Ukraine's interior minister says at least three pro-Russian separatists were killed and 13 wounded in a clash with national guardsmen at a base in the Ukrainian town of Mariupol.
Arsen Avakov says the separatists attacked the base with grenades and gasoline bombs. He said the troops disarmed most of the attackers and arrested 63.
The incident comes as the Russian and Ukrainian foreign ministers get ready for emergency talks in Geneva with U.S. Secretary of State John Kerry and European Union foreign policy chief Catherine Ashton.
The White House says Kerry will look for any signs Russia is serious about de-escalating the tensions in Ukraine.
Spokesman Jay Carney says the United States is prepared to impose new sanctions on Russia.
In an interview Wednesday with CBS television, President Barack Obama said there will be consequences each time Russia takes steps to destabilize Ukraine and violate its sovereignty. But Mr. Obama said he is convinced Russia is not looking for a war.
Moscow has said it has the right to protect Russian speakers in Ukraine. It accuses the new Ukrainian leadership of being anti-Russian and anti-Semitic and of threatening the rights of pro-Russians.
But senior U.N. human rights official Ivan Simonvic told the Security Council Wednesday that during two trips to Ukraine in March, his team found no widespread attacks against ethnic-Russians.
Moscow calls the U.N. report biased and unfounded.
U.S. Ambassador Samantha Power dismissed the Russian criticism, saying there is substantial evidence of Russian involvement in the unrest in eastern Ukraine. She called it a well-orchestrated professional campaign of incitement and sabotage.
Russian speakers have taken over Ukrainian government buildings in nearly a dozen eastern towns and cities. The Ukrainian army has launched what it calls an anti-terrorist operation to retake the buildings, but it is unclear how much fighting is actually taking place.