The pact – which would bind the three countries more closely to the West both economically and politically – is at the heart of the crisis in Ukraine.
Russian President Vladimir Putin said making Ukraine choose between Russia and the EU would split it in two.
A ceasefire with pro-Russian rebels in east Ukraine is due to end on Friday.
Mr Putin called for a long-term ceasefire to allow talks between the government and separatists.
Meanwhile the United Nations refugee agency said there had been a sharp rise in the numbers of displaced people in eastern Ukraine in the past week, with 16,400 people fleeing the area.
