Heavy fighting has erupted between Ukrainian government forces and pro-Russian separatist rebels near the war-torn eastern town of Krasnyy Liman.
News of the fighting came after President Petro Poroshenko talked of declaring a unilateral truce in the near future.
He said on Thursday that Ukraine would sign a historic association agreement with the EU on 27 June.
Meanwhile, Nato said Russia had moved troops back to the Ukrainian border.
Its secretary-general, Anders Fogh Rasmussen, said at least a few thousand soldiers had been deployed there, in addition to existing forces.
“I consider this a very regrettable step backwards and it seems that Russia keeps the option to intervene further,” Mr Rasmussen said in London.