Mikerevic said on Friday that he was resigning for the good of Republika Srpska, referring to the Serb-run part of Bosnia - created at the end of the former Yugoslav republic's 1992-1995 conflict.

International High Representative Paddy Ashdown, who has far-reaching powers under the 1995 Dayton peace accords, sacked six Serb police officers and three officials on Thursday.

It was his second such move this year after earlier sacking several senior Serb officials due to their refusal to cooperate with the UN war crimes tribunal at The Hague.

Karadzic still free

Several Serb suspects are still at large in the Balkans, including wartime Serb leader Radovan Karadzic, who remains on the run almost a decade after the war.

International forces have been
unable to capture Karadzic

Karadzic and his former military commander, Ratko Mladic, have been charged with crimes including genocide, notably relating to their suspected roles in the siege of Sarajevo and the massacre of about 8000 Muslims at Srebrenica in 1995.

Bosnian Serb authorities only recently apologised for the massacre, the worst atrocity in Europe since the second world war, and they have so far failed to arrest a single war crimes suspect sought by UN prosecutors.

In a letter to Bosnian Defence Minister Nikola Radovanovic, Ashdown said there were reliable reports that Serb army officers had entertained Mladic as recently as July.

Assets frozen

The US Treasury froze the US-based assets of the Serb Democratic Party (SDS), two companies and suspects, while the Department of State reiterated the need for a "special effort" to strip the fugitives of their support.

US ambassador Douglas McElhaney also announced a unilateral visa ban against leaders of the SDS and its junior partner in the Bosnian Serb government, the Party of Democratic Progress (PDP).

Former commander Ratko Mladic
remains at large

He did not name those who would be affected but they could include Foreign Minister Mladen Ivanic, who heads the PDP, and possibly also Mikerevic who is a senior member of that party.

Bosnian Serb leaders frequently warn of political chaos resulting from foreign pressure over the war crimes issue, but Mikerevic said he would remain in office until a new prime minister is in place.

US 'blackmail'

On Thursday, Mikerevic angrily accused Ashdown of blackmail, and insisted that the Republika Srpska was determined to cooperate with the UN court at The Hague.

Ashdown has not shied away from using the extensive powers of the high representative to force change in the mountainous Balkan republic of just over two million people, which is still struggling to cope with the legacy of the brutal inter-ethnic war between Catholic Croats, Muslim Bosnians and Orthodox Serbs.

The new EU peacekeeping force in Bosnia, which took over from a Nato mission earlier this month, launched raids on a vast Serb military bunker on Thursday in its first major operation to track down war crimes fugitives.