KYIV, Ukraine — As Western allies rush heavier weapons to help Ukraine reclaim occupied territory, Moscow’s forces are intensifying assaults along the eastern front in what President Volodymyr Zelensky of Ukraine has described as the opening moves of a new Russian offensive.

Both sides have been readying for heavier ground combat for months, with Moscow expected to press on with its goal of capturing the entire Donbas region of eastern Ukraine and Kyiv aiming to drive Russian troops out of the country completely.

Now, with Russia pounding away with artillery at a rate not recorded since September, and dispatching tens of thousands of soldiers to test Ukrainian defenses up and down a 140-mile stretch of the front line in Donbas, Mr. Zelensky said that Russia’s intensified assault was an effort to seize the initiative.

“Russia really wants some kind of big revanche,” Mr. Zelensky said this week. “I think it has started.”

Andriy Yusov, who represents the intelligence department in Ukraine’s ministry of defense, said he expected the fighting to intensify in February and March. “We are on the eve of a very active phase,” he said in an appearance on Ukrainian national television.

Ukraine and Russia have been locked in a grueling combat for nearly a year. Since the fall, when Ukraine reclaimed territory in counteroffensives in the northeast and south, the fight in the east has congealed into muddy and frozen trenches, with each army causing heavy losses for the other side while managing only negligible gains.

Yet since the Kremlin named Gen. Valery V. Gerasimov to take over its struggling war effort last month, Russia has steadily added forces in Donbas, Ukrainian military officials say. Ukrainian intelligence estimates that Russia now has more than 320,000 soldiers in the country — roughly twice the size of Moscow’s initial invasion force. Western officials and military analysts have said that Moscow also has 150,000 to 250,000 soldiers in reserve, either training or being positioned inside Russia to join the fight at any time.

“We see that they are preparing for more war, that they are mobilizing more soldiers, more than 200,000, and potentially even more than that,” NATO’s secretary general, Jens Stoltenberg, told reporters during a visit to South Korea on Monday. “They are actively acquiring new weapons, more ammunition, ramping up their own production, but also acquiring more weapons from other authoritarian states like Iran and North Korea.”

A surge in Russian bombardment has accompanied the buildup of forces. Konrad Muzyka, a military analyst for Rochan Consulting, which tracks Russian deployments, said that reported Russian artillery barrages had risen from an average of about 60 per day four weeks ago to more than 90 per day last week, with 111 Ukrainian locations targeted on one day alone.

He also said that “the Russians are withdrawing a lot of equipment from storage areas.” Still, he concurred with other analysts who say that Russia will struggle to outfit large numbers of new soldiers with tanks, armored vehicles and other effective equipment.

How the Kremlin will ultimately deploy its tens of thousands of new fighters is also a matter of speculation.

Moscow could be preparing to open a new front, pushing across the Russian border to recapture territory in Sumy or Kharkiv in northeastern Ukraine after being driven out months ago, according to Ukrainian officials and military analysts. It might be escalating fighting along the eastern front to divert Ukrainian resources and hurt Kyiv’s ability to launch its own offensive. It could be planning a drive from occupied territory in eastern Ukraine to push deeper into the Luhansk and Donetsk regions, which make up Donbas.

The only matter of consensus is that Russia is not satisfied with the territory it has taken and is maintaining its ultimate goal of subjugating Ukraine. The intensified assault has continued Russia’s pattern for nearly a year: bleeding the Ukrainian military through relentless attacks.

Oleksii Danilov, the head of Ukraine National Security and Defense Council, told Sky News on Tuesday that he did not rule out “any scenario in the next two or three weeks.”

“The main fights are yet to come,” he said.

Andrew E. Kramer contributed reporting.