Word of the European oil embargo came amid a surge of activity to provide Ukraine with more weapons and support, while shoring up NATO’s defenses, as the Kremlin and Western allies seemed to gird for a drawn-out struggle that risked spilling over Ukraine’s borders.

President Biden’s request Thursday for Congress to approve $33 billion to bolster Ukraine’s arsenal and economy was followed by more commitments by allies. Britain’s military said on Friday that it would deploy 8,000 soldiers to Europe, who were to join tens of thousands of troops from NATO countries in exercises meant to deter further Russian aggression.

While the NATO allies’ commitments to Ukraine grew, the Russian offensive in the Donbas region of eastern Ukraine showed signs of stalling amid heavy battlefield losses and was now “several days behind” schedule, a senior Pentagon official said on Friday.

Britain’s Defense Intelligence agency largely concurred, saying on Friday that “Russian territorial gains have been limited and achieved at significant cost to Russian forces.”

In a video released on Friday, an aide to the Ukrainian president, Volodymyr Zelensky, called the Russian losses “colossal.”

The Russian military is trying to encircle Ukrainian troops in the Donbas region by attacking from the north, east and south, but has made little progress, experts and Pentagon officials say.

Victory in the Donbas campaign is vital to Moscow’s plans of carving out a large chunk of southern and eastern Ukraine, from Odesa in the south through Mariupol and up to Kharkiv in the north, and bringing it under Russian domination or even outright annexation.

Moscow now has 92 battalion groups fighting in Donbas — up from 85 a week ago, but still well below the 125 it had in the first phase of the war, the Pentagon official said. Each battalion group has about 700 to 1,000 troops.

Russia still has massive firepower in the region, but many of those battalions were badly damaged in early fighting around the capital, Kyiv, and have been rushed back into action in Donbas before being restored to full fighting strength, the Pentagon official said.

Some military experts gave a grimmer assessment of Russia’s prospects on Friday. Dr. Mike Martin, a visiting fellow in war studies at King’s College London, told the BBC that Russia’s offensive had “sort of fizzled” and that the battle for eastern Ukraine could be over in two to four weeks.

Russia’s early failures, its inability to do “some bold maneuver” in recent fighting and Ukraine’s growing prowess on the battlefield is behind a “major strategic shift” among Western countries, he said, as they expand their aims beyond defending Ukraine to defeating Russia and degrading its military.

In an effort to shore up its forces, Russia has unleashed a barrage of missile and artillery strikes all along the front, continuing its strategy of targeting civilian as well as military targets. “It’s brutality of the coldest and the most depraved sort,” the Pentagon spokesman, John Kirby, told reporters on Friday.

Ukrainian troops on Friday staged a counterattack in the northern Donbas, retaking Ruska Lozova, a town of around 6,000 people about 12 miles north of Kharkiv that had been occupied by Russian forces since March.

Many of the town’s remaining residents quickly evacuated, taking advantage of the now-open road to Kharkiv. Cars, some riddled with bullet holes, limped into the city, fully packed with luggage, people and pets.

The battle for Ruska Lozova is part of a broader campaign launched by Ukrainian forces in recent weeks to push Russian troops away from Kharkiv, and hopefully put it outside of Russian artillery range. Fighting has been fierce, as the Russian border is roughly 20 miles from the city.

Before the war, Kharkiv was Ukraine’s second-largest city with a population of around 1.4 million people. But it is now a shell of itself, with many of its neighborhoods emptied, after relentless bombardment.

In another sign of Moscow’s sense of urgency, several of the dozen battalion groups that had been fighting in Mariupol were sent to fight in Donbas, the Pentagon official said, even as Ukrainian fighters resisted in the beleaguered city.

The remaining Russian forces continued to pound Mariupol in their struggle to eliminate the last pocket of resistance there. The city’s mayor made a desperate appeal to the international community Friday to save those still trapped at an enormous steel plant that has become the last holdout for Ukrainian fighters and civilians.

Vadym Boychenko, Mariupol’s mayor, said there were more than 600 wounded — including soldiers and civilians — at the Azovstal complex. “They have been there for more than 60 days and they are begging to be saved,” he said, reiterating that supplies of water, medicine and ammunition were quickly depleting. “It is not a matter of days, it’s a matter of hours.”

About 20,000 civilians have been killed, he said, but denied that the city had been fully conquered.

The European Union move to ban Russian oil imports, a long-postponed step that has divided the bloc’s members and highlighted their dependence on Russian energy sources, was another sign that Ukraine’s Western allies were dialing up their support by taking difficult measures to punish Russia.

It has taken weeks for E.U. countries to agree on the contours of the measure, and intensive talks will continue over the weekend before the European Commission, the bloc’s executive, puts a finalized proposal on paper for E.U. ambassadors to approve, several E.U. officials and diplomats involved in the process said.

The diplomats and officials spoke on condition of anonymity because they were not authorized to speak publicly on the progress of the sensitive talks.

Russia is Europe’s biggest oil supplier, providing about one quarter of the bloc’s yearly needs, according to 2020 data, about half of Russia’s total exports. As the oil embargo is phased in, officials said the bloc would seek to make up the shortfall by increasing imports from other sources, like Persian Gulf countries, Nigeria, Kazakhstan and Azerbaijan.

That the European Union is now seemingly able to hammer out a compromise among its 27 member countries on a measure this difficult highlights a fundamental miscalculation by President Vladimir V. Putin of Russia in his assault on Ukraine: Instead of sowing discord, the war has forged a united front that is making tough compromises easier to reach.

“More important than the oil embargo is the signal that Europe is united and taking back the initiative,” said Mujtaba Rahman, managing director for Europe at Eurasia Group, a consultancy. Mr. Rahman said that a more abrupt cut to oil imports would have been more painful for Russia, but also too costly for Europe, risking erosion of public support for Ukraine.

If enacted next week, as expected, the oil embargo will be the biggest and most important new step in the E.U.’s sixth package of sanctions since Russia invaded Ukraine. It will also include sanctions against Russia’s biggest bank, Sberbank, which had so far been spared, officials said.

Germany’s position has been critical in finalizing the new measure; the country, the bloc’s economic leader, was importing about a third of its oil from Russia at the time of the Ukraine invasion. But its influential energy minister, Robert Habeck, said this week that Germany had been able to cut that to just 12 percent in recent weeks, making a full embargo “manageable.”

“The problem that seemed very large for Germany only a few weeks ago has become much smaller,” Mr. Habeck told the news media during a visit to Warsaw on Tuesday. He added, “Germany has come very, very close to independence from Russian oil imports.” But he did not explain how it was able to accomplish that so quickly.

Matina Stevis-Gridneff reported from Brussels and Thomas Gibbons-Neff from Kharkiv, Ukraine. Eric Schmitt contributed reporting from Washington, and Cora Engelbrecht from Krakow, Poland.