Two people have been killed in clashes between pro-Ukrainian and pro-Russian activists in Ukraine’s eastern city of Kharkiv, officials say.
Five people were injured overnight, as gunshots were fired. Rival groups blamed each other for the violence.
Earlier, Russia and the US failed to agree on how to resolve the crisis in Ukraine’s Crimea region, ahead of a secession referendum there.
Russia vowed to respect Sunday’s vote – but the US said it was illegitimate.
Moscow has been tightening its military grip on Crimea – the southern autonomous republic in Ukraine – where voters are to decide on whether to re-join Russia or stay with Kiev.
Also, the UN Security Council is to vote on Saturday on a US-drafted resolution that defines Crimea’s referendum as illegal.
US Vice-President Joe Biden will travel to Poland and Lithuania early next week to discuss ways to support Ukraine’s sovereignty, as well as Nato members’ reciprocal defence commitments.
The Pentagon says it will keep its aircraft carrier battle group in the Mediterranean Sea for several days longer than planned because of the Ukraine crisis.
Russia has moved a column of army trucks and a number of artillery pieces into northern Crimea, eyewitnesses say.
Ukrainian border guards begin checks on trains coming from Crimea into the rest of Ukraine.
The violence reportedly began on Kharkiv’s Svoboda Square on Friday evening and later moved to an office of a pro-Ukrainian group in the city.