As Ukraine, Georgia and Moldova signed partnership agreements with the European Union Friday, Russia said the move will have "serious consequences".
Russian President Vladimir Putin said making Ukraine choose between Russia and the EU would split it in two, BBC reported.
The agreement would allow the three nations to work more closely with the West.
Ukrainian President Petro Poroshenko, however, described the signing of the agreement as his country's most historic day since independence in 1991, and called it a "symbol of faith and unbreakable will".
European Council (EC) President Herman van Rompuy described it as a "great day for Europe".
Former Ukrainian president Viktor Yanukovych had refused to sign the deal under pressure from Russia and the protests that followed led to his overthrow in February.
Talks are also set to take place on extending a ceasefire for pro-Russia rebels which was announced by Poroshenko last week in the eastern regions of Donetsk and Luhansk.
