Ukraine, Georgia and Moldova have signed partnership agreements with the European Union, in a move strongly opposed by Russia.
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.
Ukrainian President Petro Poroshenko, in Brussels to sign the pact, said he would take a decision on an extension to the truce when he arrived back in Kiev later on Friday.
Russian Foreign Minister Sergei Lavrov said he would welcome an extension, but not if it were simply an ultimatum for separatists to lay down their arms.
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.
The total number internally displaced has reached 54,400, while a further 110,000 people left Ukraine for Russia this year.
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