KYIV — After taking the strategic northeast Ukrainian town of Avdiivka two weeks ago, Russian forces have seized three more villages in the past few days, suggesting a growing momentum in their advance even as Western officials warn of the ammunition shortages Kyiv’s military is facing.

Russia’s Defense Ministry announced Wednesday that its troops had taken the village of Stepove, seven miles northwest of Avdiivka. Ukrainian officials said the previous day that Kyiv’s forces had pulled back from Stepove and the neighboring village of Sieverne.

Ukrainian forces also withdrew from the village of Lastochkyne “to organize defenses” along a new line of settlements, “aiming to prevent further enemy advancement to the west,” Dmytro Lykhoviy, a military spokesman, said Monday on Ukrainian television.

The villages had little strategic importance, and Stepove and Sieverne had populations of less than 100 people even before Russia’s invasion, two years ago. But the gains indicated that Russia was pushing its advantage after taking Avdiivka — its first major victory since seizing the eastern city of Bakhmut last May.

Ukraine’s situation was “undoubtedly a tough one,” made more acute since its military was “struggling with its ammunition and its stockpiles,” Adm. Sir Tony Radakin, the head of Britain’s armed forces, said at a conference in London on Tuesday.

“At a tactical level, you’re seeing some Russian success, gaining relatively small amounts of territory,” he said. “I think that’s a predicament that is likely to last at least for the next few months.”

House Republicans are blocking some $60 billion in U.S. military assistance to Ukraine, part of a larger package that includes aid to Israel and Taiwan. A meeting Tuesday between President Biden and congressional leaders failed to break the impasse.

U.S. officials attribute the loss of Avdiivka to the shortage in ammunition and warn that circumstances could become markedly worse if Congress does not approve the aid package.

“The situation is extremely serious right now,” State Department spokesman Matthew Miller said Tuesday. “We have seen Ukrainian front-line troops who don’t have the ammo they need to repel Russian aggression. They’re still fighting bravely. They still have armor and weapons and ammunition they can use, but they’re having to ration it now because the United States Congress has failed to act.”

Russian forces had “started taking some other towns and villages,” White House spokesman John Kirby said on Tuesday. “They’re on the move.”

“The situation is very dire,” he said, adding, however, that he was “not in a position to put a time stamp on it and say by such and such date [the Ukrainians] will lose the war.”

“But they are certainly beginning to lose territory — territory that they had clawed back from the Russians and now they have to give it back to the Russians because they can’t, they can’t fight them off,” Kirby said.

Serhiy Morgunov and Serhii Korolchuk contributed to this report.