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.
In other developments:
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