Ukrainian interim Interior Minister Arsen Avakov said more than 30 separatists were killed in fighting in Slovyansk. A number of Ukrainian servicemen were killed and wounded.
"Four of our fighters were killed and 20 were wounded. According to our estimates, over 30 terrorists were killed and dozens were wounded," Avakov said on his Facebook page on Tuesday, adopting the term Kyiv uses for the pro-Russian insurgents in control of Slovyansk.
Avakov said the fighters included several from Russia and Chechnya as well as Crimea, annexed in March by Moscow.
Ukraine's forces have created a security cordon around the city and on Monday moved in in an attempt to squeeze the pro-Russian forces. Militants downed a Ukrainian helicopter gunship near the town, although the pilots were reported to have survived.
The army's offensive on the city began last Friday, killing at least nine people, including two Ukrainian servicemen who died when rebels shot down two army helicopters using surface-to-air missiles.
Kyiv has also moved to restore its grip on the southern port city of Odessa, sending in an elite national guard. The area had been largely peaceful until last Friday, when clashes killed over 40 people, many of them in a government building that was set on fire.
It comes as thirty foreign ministers, including from Russia and Ukraine, meet in Vienna on Tuesday to debate the crisis at a Council of Europe meeting.
In an interview with European newspapers published on Tuesday, German Foreign Minister Frank-Walter Steinmeier warned that more needed to be done to prevent a new Cold War.
"The Ukraine conflict has increased in speed and intensity, in a way we would not have thought possible some time ago," Steinmeier told Spain's El Pais, France's Le Monde, Italy's La Repubblica and Poland's Gazeta Wyborcza.
The German foreign ministry has advised all Germans to leave the south and east of Ukraine.
jr/kms (AP, Reuters, AFP)