By Ralph Boulton
KIEV, May 9 (Reuters) - Ukraine's interior minister said security forces had killed about 20 pro-Russian rebels in the port city of Mariupol on Friday, in what appeared one of the biggest actions in Kiev's attempt to end an insurgency in the country's east.
Interior Minister Arsen Avakov said an attempt by "terrorists" to seize police headquarters turned into a pitched battle inside the building with Ukrainian army, national guard and security forces.
The Ukrainian forces brought armored personnel carriers into the town and the building caught fire at one point. The rebels then fled into the town.
The clash came amid international attempts to forge some form of settlement in Ukraine to prevent a slide into a civil war that could have dramatic consequences for the whole region.
Mariupol, an important industrial and shipping centre in the Donetsk region that is planning to hold a secessionist referendum on Sunday, has been the focus of frequent skirmishes in recent days. But Friday's action appeared to be a concerted effort by several forces to claw back control from rebels.
"A terrorist group of about 60 men armed with automatic weapons attacked the police headquarters...About 20 terrorists were destroyed and four taken prisoner," Avakov said on his Facebook page.
"To those who come with weapons and who shoot and who take hostages, torture them, rob people, hiding behind various slogans - to them there can be only one answer from the Ukrainian state - annihilation."
The death toll could not be confirmed independently and each side has tended to exaggerate the other's losses during clashes.
If confirmed, it would be among the heaviest tolls inflicted on the rebels during fighting. Kiev says the separatists are backed by Moscow and by Russian special forces on the ground but Russia denies this.
Avakov seemed to appeal to the local population to back the Ukrainian forces.
"Everyone decides for himself. The situation has become black-white...Enough politicking, foolish behavior and posturing," he said.
Avakov made particular mention of the broad involvement of several arms of the security forces including special forces, National Guard and Army. One of the weaknesses of what Kiev calls its "Anti-Terrorist Operation" in the east has been a failure of various units to coordinate. (Editing by Timothy Heritage)