Several casualties in fight with pro-Russian group

Violence in Slavyansk threatens to escalate tensions with Moscow

Kiev said on Sunday that several people have been left "dead and wounded" in fighting to oust pro-Russian gunmen holed up in a police station in Ukraine's restive east, as Washington warned Moscow to de-escalate the crisis or face the consequences.

Interior Minister Arsen Avakov said both sides had suffered casualties in the offensive in Slavyansk that threatens to further escalate tensions with Russia, which has 40,000 troops massed on Ukraine's eastern border and which has warned Kiev against the use of force.

People gesture while pro-Russian protesters gather at the police headquarters, with a military helicopter flying above, in Slavyansk, Ukraine, on Sunday. Ukraine's government said it has sent security forces into the eastern city where pro-Russian militants have seized control in what Kiev describes as an act of aggression by the Kremlin. Gleb Garanich / Reuters

"On our side - an SBU (Ukrainian Security Service) officer. On the side of the separatists - an unidentified number. ... The separatists have started to protect themselves using human shields," Avakov wrote on his Facebook page.

Helicopters hovered low over the poor mining town, where a thick column of black smoke could be seen.

Residents, mostly women, huddled in the cold under light rain in front of barricades protecting the police building. Armed separatists have also set up a checkpoint at the entry to the town.

Earlier, Avakov announced that units from "all of the country's force structures" were taking part in the first tough response from Kiev to the unrest sweeping the volatile eastern part of the country. He said the gunmen had opened fire on Ukraine's special forces and were "shooting to kill".

This came after a week of soaring tensions as pro-Russians demanding greater autonomy, or to join nearby Russia, stepped up protests in the region ahead of May 25 presidential polls.

The protesters refuse to recognize the new pro-Western government in Kiev, which swept to power on the back of bloody winter protests against fallen president Viktor Yanukovych's decision to reject closer ties with the European Union and move closer to Russia.

On Saturday, US Secretary of State John Kerry phoned his Russian counterpart, Foreign Minister Sergey Lavrov, and "made clear that if Russia did not take steps to de-escalate in eastern Ukraine and move its troops back from Ukraine's border, there would be additional consequences", a senior State Department official said.

US Vice-President Joe Biden will travel to Kiev on April 22, becoming the most senior US official to visit the country since the crisis began there.

A wave of US sanctions unveiled in March blacklisted officials and businesspeople close to Russian President Vladimir Putin to protest Moscow's takeover of Crimea.

On Sunday, France said it would support new sanctions against Moscow if there were a military escalation in Ukraine, speaking on the eve of a meeting of EU foreign ministers in Luxembourg to discuss the crisis.

On late Saturday, UN Secretary General Ban Ki-moon called on all sides to "exercise maximum restraint" and engage in dialogue to calm a situation that has a "growing potential for violent clashes", a UN statement said.

And separatist protesters seized control on Sunday of the mayor's office in the town of Mariupol, eastern Ukraine, on the Azov Sea, local media said.

The protesters entered the building after a rally involving about 1,000 people demonstrating in favor of the creation of a separate republic in eastern Ukraine, a local journalist for the newspaper Priazovsky Worker said.

AFP-Reuters