Ukraine, Georgia and Moldova have signed partnership agreements with the European Union, in a move strongly opposed by Russia, BBC News reports.
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.
Ukrainian President Petro Poroshenko said he saw the signing as the start of preparations for EU membership. A ceasefire with pro-Russian armed groups in the east is due to end on Friday, June 27, although fighting is said to have continued in some areas despite the truce.
Poroshenko hailed the signing as Ukraine's most historic day since independence in 1991, describing it as a "symbol of faith and unbreakable will".
Meanwhile European Council President Herman van Rompuy described it as a "great day for Europe".
"The EU stands by your side, today more than ever before," he told leaders of the three countries, adding that there was nothing in the agreements that might harm Russia in any way.
Poroshenko's predecessor Viktor Yanukovych refused to sign the deal under pressure from Russia and protests led to his overthrow.
Talks on extending the truce in the Donetsk and Luhansk regions are also set to take place on Friday.