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.
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
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.
Eyewitnesses said that pro-Russian activists tried to storm the rival protesters, who had barricaded themselves in.
The witnesses said that shots had been fired and Molotov cocktails thrown in.
Kharkiv Mayor Hennadiy Kernes was later quoted by Ukrainian media as saying that two people were killed and five injured.
Meanwhile, Kharkiv Governor Ihor Baluta called the incident "a provocation".
Both rival groups blame each other for starting the clashes. A criminal investigation is now under way.
This follows Thursday night's violence in Donetsk, also in the east, where at least one person died in fighting between a pro-Russian crowd and supporters of the new government in Kiev.
Ukraine accuses Russia of using provocateurs to stoke unrest on the eastern border. Moscow denies this, vowing to protect its "compatriots" from far-right radicals.
Earlier on Friday, US Secretary of State John Kerry and Russian Foreign Minister Sergei Lavrov failed to reach agreement on how to resolve the Ukraine crisis.
After six hours of talks in London, Mr Lavrov said both sides had "no common vision" on the issue.
However, he described the meeting as "constructive".
And Mr Lavrov stressed that Moscow would "respect the will of the people of Crimea".
Meanwhile, Mr Kerry, who described the talks as "direct and candid", said the US acknowledged Russia's "legitimate interests" in Ukraine.
But he stressed that Washington had not changed its position on the "illegitimate" referendum in Crimea and would not recognise its outcome.
He also said that his Russian counterpart had made it clear that President Vladimir Putin was not prepared to make any decision until after the vote.
And Mr Kerry added that he had told Mr Lavrov that there would be consequences if Russia "does not find a way to change course".
Both the US and EU have threatened tough sanctions against Moscow.
Russia's military intervention in the Crimean peninsula - part of Russia until 1954 and host to its Black Sea fleet - followed the fall of Ukraine's pro-Moscow President Viktor Yanukovych on 22 February.