Ukraine sent an elite national guard unit to its southern port of Odesa, desperate to halt a spread of the fighting between government troops and a pro-Russia militia in the east that killed combatants on both sides Monday.
The government in Kyiv intensified its attempts to bring both regions back under its control, but seemed particularly alarmed by the bloodshed in Odesa. It had been largely peaceful until Friday, when clashes killed 46 people, many of them in a government building that was set on fire.
The tensions in Ukraine also raised concerns in neighbouring Moldova, another former Soviet republic, where the government said late Monday it had put its borders on alert. Moldova's breakaway Trans-Dniester region, located just northwest of Odesa and home to 1,500 Russian troops, is supported by Moscow, and many of its residents sympathize with the pro-Russia insurgency.
The loss of Odesa — in addition to a swath of industrial eastern Ukraine — would be catastrophic for the interim government in Kyiv, leaving the country cut off from the Black Sea. Ukraine already lost a significant part of its coastline in March, when its Crimean Peninsula was annexed by Russia.
Compared with eastern Ukraine, Odesa is a wealthy city with an educated and ethnically diverse population of more than 1 million. Jews still make up 12 per cent of the population of the city, which once had a large Jewish community.
"The people of Odesa are well-educated and understand perfectly well that Russia is sowing the seeds of civil war and destabilization in Ukraine," said Vladimir Kureichik, a 52-year-old literature teacher who left Crimea after it became part of Russia.
The White House said it was "extremely concerned" by the violence in southern Ukraine.
"The events in Odesa dramatically underscore the need for an immediate de-escalation of tensions in Ukraine," said spokesman Jay Carney. He suggested Russia still must follow through with its part of a diplomatic deal aimed at defusing the tensions.