Seeking to dominate the strategic waterways of Asia, China has deployed an armada of boats that are equipped with 76-millimeter cannons and anti-ship missiles, and are bigger than U.S. Navy destroyers. But they are not Chinese Navy vessels. Their hulls are painted white, with “China Coast Guard” in block letters on the sides.

In just a decade, China has amassed the world’s largest coast guard fleet, and it is like no other. More militarized, more aggressive in international disputes and less concerned with the usual missions of policing smugglers or search and rescue, the Chinese force has upended 200 years of global coast guard tradition.

It has also set off an arms race. Powering into a gray zone between law enforcement and naval power, Beijing has targeted rivals with ships that can easily sink the vessels most coast guards have used for decades. And in response, other countries that fear Chinese encroachment are rushing to deploy bigger, more heavily armed patrol boats of their own.

The waters around Taiwan, the self-governed island China claims as its own, are one potential battleground. But with coast guard standoffs quietly escalating around the region, officials and analysts increasingly worry about a rising threat: an accident or violent skirmish anywhere in the vast area that China’s Coast Guard roams, which could spark a broader conflict, even a war between major powers.

From March 30 to April 2, a squadron of Chinese Coast Guard ships circled the contested islands that Japan calls the Senkakus for 80 hours and 36 minutes — China’s longest-ever stay, according to maritime data.

Japan later announced a plan to upgrade its coast guard and fold it into the Ministry of Defense.

Two more recent incidents also point to new levels of Chinese assertiveness and regional risk:

• Starting around April 8, Chinese patrol ships crowded near Taiwan, threatening for the first time to stop and search Taiwanese vessels during Chinese military drills prompted by a meeting between President Tsai Ing-wen and the House speaker, Kevin McCarthy. Taiwan is now developing plans to pierce any future blockades while hardening its own coast guard.

• On April 23, near a disputed shoal in the South China Sea, one of China’s large cutters maneuvered into the path of a much smaller Philippine patrol boat, forcing its captain to throw its engines into reverse to avoid a collision. A few days later, the United States promised to give the Philippines six new upgraded patrol vessels.

These altercations — along with additional Chinese incursions near Vietnam and the Pacific Island nation of Palau in May and June — fit a pattern of intensifying tensions, marking a major shift in how nations claim territory and protect their interests in the world’s oceans. Coast guards that once acted as watchful eyes and helping hands have become more like navies, drawn into Asia’s geopolitics and deployed as military muscle in waterways that are vital for shipping and natural resources.

From ports in southern China and Taiwan to American bases in Guam, white-hulled coast guard vessels are getting longer and heavier, or smaller and faster. Their guns are also getting bigger, or they are being built to allow for complex weapons systems to be bolted on at a moment’s notice. And the region’s coast guards are working more closely with defense planners, putting them at the vanguard of broader contests in the Indo-Pacific over economic and military power.

“This is not how it was 10 years ago,” said John Bradford, a retired U.S. Navy commander and senior fellow in the Maritime Security Program at the S. Rajaratnam School of International Studies in Singapore. “Many countries across the region have started using their coast guards to assert sovereignty.”

“The idea,” he added, “is that it’s more effective because you’re less likely to push up the escalation ladder because they’re lightly armed. But when a coast guard vessel gets missiles on it, how is it different from a navy vessel except for the color of the paint on the hull?”

The coast guard competition now emerging in Asia began with China’s push to become what it called a “maritime great power.”

That phrase, setting out a national priority, appears in Chinese government documents as far back as 2000, with a definition that includes naval power, fishing prowess, environmental protection and the advancement of territorial claims. The coast guard’s leading role was solidified in 2013 under Xi Jinping, who, in his first year as China’s leader, created the seagoing force by consolidating five agencies.

The coast guard, in China’s eyes, would be a pillar of its rejuvenation as a world power because it would help Beijing control important waterways (and their fishing and mining riches) without spurring a military response from countries flummoxed by the fleet’s not-quite-military heft.

What followed were dozens of confrontations confirming that China’s coast guard — often working with a militia of fishing and other kinds of vessels — could patrol, prod and intimidate rivals with near impunity.

In 2013, there were several tense standoffs in the South China Sea between Chinese Coast Guard vessels and Filipino troops occupying a World War II-era ship called the Sierra Madre.

In 2014, in the same sea off the coast of Vietnam, a Chinese Coast Guard ship rammed a Vietnamese Coast Guard vessel after Vietnam tried to stop China from building an oil rig in contested waters.

In 2016, China’s coast guard rammed free a fishing boat that had been seized by the Indonesian authorities.

More recently, China has expanded both the mission and the fighting capacity of its fleet. A 2021 law grants its coast guard — which falls under military control — the right to use lethal force against foreign ships in waters that Beijing claims, including the South China Sea, where it has built forward operating bases on artificial islands.

Regional experts say the provisions violate international law by allowing China’s coast guard, without declaring war, to engage in warlike behavior beyond its national jurisdiction.

And its boats increasingly have the power to do so. China now has around 150 large coast guard patrol ships of at least 1,000 tons, compared to roughly 70 for Japan, 60 for the United States and just a handful for most countries in Asia. The Philippines has 25 patrol ships to deploy in the South China Sea. Taiwan’s coast guard consists of 23 boats, according to U.S. officials.

Many of China’s coast guard vessels are former navy corvettes, capable of long-endurance operations and equipped with helicopter pads, powerful water cannons and guns the same caliber as those on an M1 Abrams tank. According to the most recent Defense Department report on China’s military, 85 of its coast guard vessels carry anti-ship cruise missiles.

This new fleet of warships dressed up as law enforcement vessels is what many countries in Asia are forced to confront almost daily as China pushes further into disputed territory, for longer periods. And it’s not just in the South China Sea.

On May 11, in the East China Sea, two Chinese Coast Guard vessels breached the 12-mile territorial limit around the Senkaku Islands for the 13th time this year. In 2022, alternating teams of 1,500-ton Chinese Coast Guard vessels spent 336 days circling the disputed islands, up from 171 in 2017, according to Japanese tracking data.

“We have confirmed some ships deployed guns,” Hiromune Kikuchi, a Japanese Coast Guard spokesman, said in an interview. “We are concerned that they have increased numbers of large ships with military capabilities.”

Increasingly, so too have the coast guards of other countries.

Vietnam ordered six large coast guard ships from Japan to be delivered by 2025.

South Korea announced last year that it would build nine new 3,000-ton patrol ships for the seas off its western coast, where the maritime boundary with China is unclear.

Japan approved a law in December that will increase its coast guard budget by nearly $1 billion — a 40 percent surge — and fold the fleet into its national defense forces.

The United States and Australia have also become more active in the Pacific with gifts of patrol boats, new maritime surveillance centers and, for the Americans, a new generation of larger Coast Guard cutters and patrol agreements with several nations — adding Papua New Guinea just in recent weeks.

The United States is also now working more closely with Japan and the Philippines in the South China Sea, conducting joint coast guard training exercises in the Philippines last year and again this June, drawing complaints from Beijing.

“The coast guards and different nations in the region are maturing,” said Vice Adm. Andrew J. Tiongson, the U.S. Coast Guard’s Pacific Area commander. “I think they’re maturing out of necessity.”

Nowhere is that dynamic more obvious than in the Taiwan Strait and the shipyards of southern Taiwan. On an island at the center of regional anxieties, Taiwan’s coast guard is expanding far more rapidly than its Navy while confronting almost daily challenges from China.

On one recent visit to an industrial area just outside the port of Kaohsiung, workers put the final touches on repairs for a coast guard patrol boat whose nose had been sheared off at sea.

“A Chinese ship hit this boat and broke right through it,” said Hun Yenlu, a former Taiwanese Navy officer who runs Karmin International, a company that builds and repairs Taiwanese Coast Guard vessels.

A few weeks earlier, he said, the patrol boat — a 36-foot rigid inflatable, similar to assault craft used by U.S. Navy Seals — had helped form a cordon with a few others around a suspicious-looking speedboat near Taiwan’s outer islands. That boat had six engines, a common design for China’s maritime militia, and when the Taiwanese Coast Guard asked about its mission, the pilot pushed the throttle and punched through.

“There was no name on that ship, but we know it was Chinese,” said Mr. Hun, recounting the story officials had told him. “When you don’t see a name, you know it’s suspicious.”

It was one of many collisions and near misses caused by aggressive Chinese tactics near Taiwan, according to maritime officials and boat builders.

On June 3, the U.S. military said that an American naval destroyer, the U.S.S. Chung-Hoon, slowed to avoid a possible collision with a Chinese Navy ship that crossed in front of the Chung-Hoon as it passed through the strait between China and Taiwan.

China’s threat in April to inspect Taiwanese vessels represented another kind of climb up the escalatory ladder. The response to it revealed the blurring lines of aggression at sea.

Taiwan’s Ocean Affairs Council said it had responded to China’s threat by employing a coast guard boat of its own as a shadowing force to “prevent mainland China from endangering the freedom of navigation and safety of our citizens.” A spokesman for Taiwan’s office overseeing relations with Beijing said: “If you interfere, we will hit back.”

A second shipyard near the port in Kaohsiung offered hints of what that might mean.

A new 100-ton patrol boat bobbed in the water with a strong steel hull rather than the lighter materials of earlier iterations, for protection in case of ramming. On one of the piers, a 600-ton coast guard vessel with a fresh coat of white paint waited for engineers to add the same radio and radar that Taiwan’s Navy uses.

On the side, there was a wide gap in the hull — for missile launchers, if needed.

Hisako Ueno contributed reporting from Tokyo, Amy Chang Chien from Taipei and Zixu Wang from Hong Kong.