Russian troops have captured or entered around a half-dozen villages on Ukraine’s eastern front over the past week, highlighting the deteriorating situation in the region for outgunned and outnumbered Ukrainian forces as they wait for long-needed US military aid.

“The situation at the front has worsened,” Gen. Oleksandr Syrsky, Ukraine’s top commander, said in a statement on Sunday in which he announced that his troops had retreated from two villages west of Avdiivka, a Ukrainian stronghold in the east that Russia seized earlier this year, and another village farther south.

Military experts say Moscow’s recent advances reflect its desire to exploit a window of opportunity to press ahead with attacks before the first batch of a new US military aid package arrives in Ukraine to help relieve its troops.

The US Congress approved $60 billion in military aid for Ukraine, and President Joe Biden signed it last week, vowing to expedite the shipment of arms. Syrsky said the “most difficult situation” at the moment was around the villages west of Avdiivka, which Russia captured in February. He said Russia had deployed up to four brigades in the area.