Russia's troops have gone back on the offensive this year, capitalising on an advantage in manpower and arms to press forward, particularly in the eastern Donetsk region.
"The situation at the front has worsened," Oleksandr Syrsky said in a Facebook post Sunday.
He said Ukrainian troops had "retreated" to new defensive lines further to the west in some areas, conceding the loss of territory to the advancing Russians.
"The enemy has concentrated its main efforts in several sectors, creating a significant advantage in forces and means," he said.
Russia had secured "tactical successes in some sectors," he admitted.
Russia's defence ministry earlier on Sunday claimed its troops had captured the village of Novobakhmutivka in the eastern Donetsk region -- around 10 kilometres (6 miles) north of Avdiivka, which it captured in February.
Kyiv's forces are outnumbered across the battlefield, with the country struggling to recruit enough soldiers to replace those who have been killed, wounded or exhausted by the war, now in its third year.
Ukraine is also awaiting the arrival of billions of dollars in US weapons it hopes will end months of ammunition shortages and allow it to stabilise the front lines.
But leaders in Kyiv have warned the military outlook could worsen before then.
Ukraine's head of intelligence at the ministry of defence Kyrylo Budanov said this month that the battlefield situation would likely be at its most difficult in mid-May to early June.
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