Ukraine's commander-in-chief has said the situation on the frontline has worsened in the face of multiple Russian attacks.
Oleksandr Syrskyi said Ukrainian forces had withdrawn from positions in the eastern Donetsk region.
Russia is trying to take advantage of its superiority in manpower and artillery before Ukrainian forces get much-needed supplies of new US weapons.
The US last week agreed a $61bn (£49bn) package of military aid for Ukraine.
But new US weapons are yet to make their way to the frontlines, where Ukrainian troops have been struggling for months with a shortage of ammunition, troops and air defences.
"The situation at the front has worsened," Gen Syrskyi said in a post on the Telegram messaging service on Sunday.
He confirmed Ukrainian forces had withdrawn from some of their positions in an area of Donetsk that had formed part of a defensive line, established after Russia captured Avdiivka in February.
Much of the fighting has been taking place around Chasiv Yar, a Kyiv-controlled stronghold which Russia has been trying to reach after seizing Avdiivka.
New defensive lines had been taken up further to the west in some areas, with Gen Syrskyi conceding the loss of territory to the advancing Russians.
Gen Syrskyi added that rested Ukrainian brigades were being rotated in those areas to replace units that had suffered losses.
Moscow had secured "tactical successes in some sectors," he said.
Newer articles
<p>The two leaders have discussed the Ukraine conflict, with the German chancellor calling on Moscow to hold peace talks with Kiev</p>