Ukraine signed a free-trade and political cooperation agreement with the European Union on Friday that has been at the heart of the country's political crisis.
Georgia and Moldova signed similar deals, holding out the prospect of deep economic integration and unfettered access to the EU's 500 million citizens.
Ukrainian President Petro Poroshenko said the deal is a result of seven years of work between the Ukraine and the EU and represents an historic turning point for his country.
"Over the last months, Ukraine paid the highest possible price to make her European dreams come true," he told EU leaders at a signing ceremony in Brussels.
Russia immediately said "grave consequences" would follow, Reuters reports.
Ukraine's former pro-Moscow President Viktor Yanukovich turned his back on signing the EU agreement in November last year in favour of closer ties with Moscow, prompting months of street protests that eventually led to his fleeing the country.
Soon afterwards, Russia annexed Ukraine's Crimea region, drawing outrage and sanctions from the United States and the EU.