Russia is exploiting delays in aid to Ukraine, and the situation in areas where Moscow's troops are concentrated is "extremely difficult", Ukrainian President Volodymyr Zelenskiy said on Monday.

Zelenskiy was speaking after visiting the frontline in the Kupiansk sector in the northeast, an area recaptured by Ukraine in 2022, but where Russian forces have been active in recent months.

His visit took place days after Russia secured control over Avdiivka, further south, following months of intense battles, its biggest frontline gain in nine months.

"There is now an extremely difficult situation in several parts of the frontline, precisely where Russian troops have concentrated maximum reserves," Zelenskiy said.

"They are taking advantage of delays in aid to Ukraine and this is a very sensitive matter. Artillery shortages, the need for frontline air defence and for longer-range weapons."

Zelenskiy has expressed confidence that the U.S. Congress will endorse a large aid package, including assistance to Ukraine, delayed by wrangling among lawmakers.

U.S. President Joe Biden he was willing to meet with House of Representatives Speaker Mike Johnson to discuss the funding bill, adding that Republicans are making a mistake by opposing the aid package.

The Senate this month passed a $95 billion aid package that includes funds for Ukraine, but Johnson has declined to bring it up for a vote on the floor of the House, which Republicans control by a 219-212 margin.