Serbian forces have launched a new offensive in southern Kosovo against ethnic Albanian rebels, Serb and Albanian sources said Sunday.

Police units and specially trained elite troops have begun mop-up operations to purge the secessionist Kosovo Liberation Army from the area, 30 miles southwest of the province's capital of Pristina, a Serb police source said on condition of anonymity.The latest action followed a strong Serb offensive against KLA rebels in central Drenica region. That offensive came as the U.N. Security Council demanded an end to the Serb crackdown and NATO finalized plans for airstrikes unless the fighting stops.

In Bosnia-Herzegovina, Defense Secretary William Cohen repeated warnings that Yugoslav President Slobodan Milosevic has only "a short period of time" to comply with international demands to end the conflict.

"A failure to do so presents the possibility that a military operation against Mr. Milosevic would be necessary," Cohen said. "A failure to do so on his part will be met with very strong NATO action in my judgment."

The separatists suffered defeats in southern Kosovo earlier this year, but apparently have regrouped. Police sources said the rebels recently kidnapped two Serbs, which prompted the new action.

The Albanian-run Kosovo Information Center said Serb forces were attacking from three directions near the towns of Suva Reka, Urosevac and Stimlje, southwest of Pristina. The attack focused on 11 villages, and three were on fire.

Some estimates have placed the number of civilians displaced by the fighting at 15,000. But an international aid official, speaking on condition of anonymity, said that figure is probably exaggerated.

Several hundred people have died and about a quarter of a million people have been displaced since the government launched a crackdown against ethnic Albanian separatists in Kosovo in February.

The continued violence in Kosovo has led Western powers to threaten Milosevic with NATO intervention unless violence ends in the restive Serbian province, where ethnic Albanians outnumber Serbs 9-1.

The alliance has finalized plans to hit strategic targets throughout Serbia, the dominant republic of Yugoslavia.

Meanwhile in Eagle Base, Bosnia-Herzegovina, NATO troops on Sunday arrested a former Bosnian Serb police chief indicted by the international war crimes court for murder, rape and other crimes during the 1992-95 Bosnian war.

Stevan Todorovic was apprehended without incident in the northern Bosnian town of Bosanski Samac, about 44 miles northwest of here, a NATO statement said.