Moscow’s forces are three miles from Dnipropetrovsk, a province they have never invaded. If they cross in, the advance would be a morale blow to Ukraine and complicate any territorial negotiations.

Trump's push for a quick peace with Moscow while sidelining Ukraine has caused fear among many Ukrainians and their allies. But front-line troops say they are determined to fight on, citing three years of bitter sacrifices and the loss of tens of thousands of comrades-in-arms.

North Korea has deployed thousands of troops to support Russian forces fighting in Ukraine, according to Ukrainian, U.S. and South Korean assessments, Pyongyang's first major involvement in a war since the 1950s.

Russia said its forces have taken back more than 800 square kilometers of territory from Ukraine in the Kursk region of western Russia, which had been taken by Ukraine since an incursion began last year.

European Union foreign policy chief Kaja Kallas said on Thursday that interactions between the United States and Russia showed Moscow had not given up its territorial goals in Ukraine, and that appeasing it would send a dangerous signal.