Ukraine has sent an elite national guard unit to its southern port city of Odessa, to stop the spread of fighting between government troops and a pro-Russia militia. Neighboring Moldova has put its borders on alert.
Kyiv drafted police special forces to Odessa, a wealthy city with an ethnically diverse population of more than one million.
It had been largely peaceful until Friday, when clashes killed over 40 people, many of them in a government building that was set on fire.
The Black Sea port remained quiet on Monday, but the Kyiv government is worried that pro-Russian tendencies may increase, after it already lost a significant part of its coastline in March, when the Crimean Peninsula was effectively annexed by Russia.
Moldova fears spillover
Concern is mounting also in neighboring Moldova which has a 1,220 kilometer (760 miles) border with Ukraine.
The government there issued a statement on Monday evening saying security forces had been put on alert and had been ordered "to take all necessary actions to ensure public order inside the country."
Moldova, a former Soviet republic, also fears for its territorial integrity: Russia supports the breakaway Trans-Dniester region, which is located just northwest of Odessa on the Black Sea and is controlled by pro-Russian separatists and home to 1,500 Russian troops
Meanwhile in Slovyansk, a city of 125,000 in eastern Ukraine near the Russian border, violence continued, (see top picture) with pro-Russian rebels reportedly shooting down a Ukrainian helicopter on Monday.
Russia has massed thousands of troops on its side of the border, prompting fears that it might move in to protect the rights of Russian-speakers.
But NATO's top military commander, General Philip Breedlove, said on Monday he did not think regular Russian troops would enter eastern Ukraine.
The Russian Foreign Ministry published a report on Monday in which it denied allegations that Moscow had played an underhand role in steering the violence. Moscow also accused the Kyiv government, which Moscow sees as illegitimate, as being dominated by "ultra-nationalists" who are responsible for rights violations on a "mass" scale.
The White House said it was "extremely concerned" by the violence in southern Ukraine.
"The events in Odessa dramatically underscore the need for an immediate de-escalation of tensions in Ukraine," said spokesman Jay Carney.
US Treasury Undersecretary David Cohen arrives in Berlin on Tuesday for talks on implementing sanctions against Russia; he will move on for talks to France and Britain.
The international community is stepping up diplomatic efforts to make sure plans for a May 25 presidential election in Ukraine can go ahead.
The Ukrainian foreign minister and his Russian counterpart arrived in Vienna to attend Tuesday's meeting of the Council of Europe.
Kyiv is keen to secure billions of dollars in loans to stave off economic collapse and hopes to secure billions from the International Monetary Fund, the US and the European Union.
Ahead of Tuesday's talks German Foreign Minister Frank-Walter Steinmeier said he was in talks with Russia, the United States, the European Union and the OSCE to hold a second peace conference in Geneva, following the failed first plan aimed at defusing the crisis, which had been signed there on April 17.
But Steinmeier also said he feared that both sides may soon be unable to get the situation under control.
"I'm convinced we are struggling against a situation that has taken on a dynamic of its own. There are groups in eastern Ukraine that are not listening to either Kyiv ... or Moscow."
Ukraine and its population of 46 million has been in turmoil since early this year when street protesters seeking closer ties with Europe caused a collapse of the Moscow-leaning Kyiv government.
rg/av (AP, AFP, Reuters)