Ukraine's top commander said on Sunday that Kyiv's outnumbered troops had fallen back to new positions west of three villages on the eastern front where Russia has concentrated significant forces in several locations.

The statement by Colonel General Oleksandr Syrskyi reflected Ukraine's deteriorating position in the east that Kyiv hopes it can stabilize once it takes delivery of U.S. weapons under a $61 billion aid package approved this week.

"The situation at the front has worsened," he wrote on the Telegram app, describing the "most difficult" areas as west of occupied Maryinka and northwest of Avdiivka, the town captured by Russian forces in February.

Kyiv's troops, he said, had taken up new positions west of the villages of Berdychi and Semenivka, both north of Avdiivka, and Novomykhailivka, further south near the town of Maryinka.

"In general, the enemy achieved certain tactical successes in these areas, but could not gain operational advantages," Syrskyi said, adding that Russia had committed four brigades to the assault.

Freshly rested Ukrainian brigades were being rotated in those areas to replace units that had suffered losses, he said.

His statement did not mention the status of Novobakhmutivka, another village near Berdychi, that Russia's defense ministry said on Sunday its forces had captured.

Moscow's troops have been slowly advancing since capturing the bastion town of Avdiivka, taking advantage of Ukrainian shortages of artillery shells and manpower.

Online battlefield maps produced by open-source intelligence analysts suggest they have advanced more than 15 km in the direction of the village of Ocheretyne since capturing Avdiivka.

Further up the front, the Kyiv-held town of Chasiv Yar is a key emerging battleground because of its position on elevated ground that could serve as a gateway to the cities of Kostiantynivka, Sloviansk, and Kramatorsk.

Syrskiy described Chasiv Yar and the village of Ivanivske to its northeast as the "hottest spots" on that part of the front. Russia's defense ministry said it had repelled Ukrainian counter-attacks near Chasiv Yar.

KHARKIV BUILDUP

In what could prove a worrying development for Ukraine, Syrskiy said his forces were closely monitoring an increase in the number of Russian troops in the area of Kharkiv, Ukraine's second largest city.

The northeastern city of 1.3 million just 30 km from the Russian border has been hammered by airstrikes in recent months in what Kyiv has said is a deliberate effort by Moscow to make Kharkiv uninhabitable.

Syrskiy said there were so signs that Russia was directly preparing for an offensive in the north of the country.

"In the most threatening directions, our troops have been reinforced by artillery and tank units," he said.

Ukraine is currently expecting a long-awaited a shipment of U.S. military aid which officials say is critical to holding off Russia's two-year-old invasion.

A Ukrainian intelligence source told Reuters this week that Russia was conducting airstrikes on Ukrainian rail lines to disrupt the delivery of U.S. weapons to the front and to complicate military logistics.