Pro-Russian separatists attacked a base of the Ukrainian national guard in an eastern city overnight and Kiev said three of the militants were killed, bloodshed likely to overshadow crisis talks in Geneva.
Ukrainian, Russian and Western diplomats arrived for the emergency talks in Switzerland, but there was little hope of any progress in resolving a crisis that has seen armed pro-Russian fighters seize whole swathes of Ukraine.
President Vladimir Putin, who overturned decades of post-Cold War diplomacy last month by declaring Russia's right to intervene in neighbouring countries and annexing Ukraine's Crimea region, may give clues to the extent of his territorial ambitions at an annual televised question and answer session. He has deployed tens of thousands of troops on the frontier.
The new deadly clashes in the port of Mariupol took place even as a modest Ukrainian military operation to recapture territory elsewhere from armed pro-Russian rebels ended in disarray, with troops surrendering rather than open fire.
Interior minister Arsen Avakov said an armed group of about 300 separatists attacked the national guard base in Mariupol with guns and petrol bombs. Three separatists were killed in shooting that followed and 13 were wounded, he said. No guardsmen were hurt.
"Given the aggressive nature of the attack on the base, an interior ministry group has been strengthened by Omega special forces. Helicopters have been sent in," he said.
Pro-Russian militants control buildings in about 10 towns in eastern Ukraine after launching their uprising on April 6. Kiev fears that Moscow, which claims the right to use military force to protect its supporters, could see any deaths as a pretext to launch an invasion.
On Wednesday, an armoured column of Ukrainian paratroops was humiliated in an attempt to retake some towns. Pro-Moscow separatists took control of some of their armoured vehicles and crowds surrounded another column, forcing the troops to hand over the pins from their rifles and retreat.
European countries and the United States are threatening Russia with more sanctions unless it takes steps at the Geneva meeting to show it will de-escalate the conflict in Ukraine.
So far, diplomacy has failed to keep up with events on the ground, with Russia's partisans seizing control of territory before Western countries can formulate a response.
Bloodshed has been limited so far during the uprising in the east, with two people killed on Sunday, including a member of the Ukrainian state security forces shot dead.
The United States and European Union have imposed visa bans and asset freezes on a small number of Russian individuals, a response that Moscow has openly mocked. However, the Western states say they are now contemplating far more serious measures that could hurt Russia's economy more broadly, which could be put into place shortly after Thursday's Geneva meeting.
"What I have said consistently is that each time Russia takes these kinds of steps that are designed to violate their sovereignty, that there are going to be consequences," US President Barack Obama said on Wednesday in an interview with CBS. Using words unheard since the Cold War, he said the United States had stronger conventional military forces than Russia, and neither side wanted a conflict.
"We don't need a war," he said.
Western countries have repeatedly made clear they are not prepared to fight for Ukraine, but the NATO alliance announced steps on Wednesday to beef up security of member states such as Poland and the Baltic countries, which feel threatened by Russian action nearby.
Upon arriving in Geneva on Wednesday, Ukrainian Foreign Minister Andriy Deshchytsia said there was still time for negotiations to ease tensions with Russia.
"I think that we still have a chance to de-escalate the situation using the diplomatic means," he said. "And we will try hard. We are trying hard - not only Ukraine - but also the United States. However, the time is now, not only to express the concerns, but to look for a more concrete and adequate response to Russia's plans and actions."
Kiev and the West believe Russian agents are directing the insurgency in the east. A US official said Washington was looking for evidence in Geneva that Russia would stop.
"The idea here is that they would stop aiding and abetting and supporting these separatists and that they would pull their troops back from the borders," the official told reporters as Secretary of State John Kerry flew to Geneva. Other US officials in Washington said they did not expect a breakthrough.
Putin has accused the Ukrainian government of risking mass bloodshed by using its military to try to crush the rebellion in the largely Russian-speaking east of the former Soviet republic.
"The sharp escalation of the conflict puts the country, in effect, on the brink of civil war," the Kremlin quoted Putin as telling German Chancellor Angela Merkel this week.
The European Commission took a step towards preparing for wider sanctions, handing documents to EU member states on Wednesday explaining the potential impact on their economies of stricter trade and financial measures, diplomats said.
The documents examine energy, finance, trade and other areas. A number of EU countries that rely heavily on Russian gas supplies are nervous about possible retaliation from Moscow and at least one EU diplomat said the measures had to be balanced.
Putin has shown no sign of backing down before his question-and-answer session. He traditionally speaks about matters close to the hearts of ordinary Russians such as dilapidated housing, inefficient local authorities and inflation.
But on the eve of the event, for which Russians had registered more than 1.5 million questions by Tuesday, presidential spokesman Dmitry Peskov said Putin would give an extensive assessment of US and EU sanctions.