The United Nations' chief prosecutor for war crimes in former Yugoslavia has called on Nato to arrest the former Bosnian Serb leader Radovan Karadzic and more than 30 other indicted war criminals still at large.

During her first talks with Nato ambassadors in Brussels, Carla Del Ponte warned that the failure to capture Mr Karadzic, who is believed to be hiding in the Serb-controlled part of Bosnia, remained a serious obstacle to lasting peace in Bosnia.

But the Secretary-General of Nato, George Robertson, said the alliance's commitment to apprehending all those indicted of war crimes was beyond question.

Four years after the end of the war in Bosnia, 20,000 Nato troops keep the peace. But they have been unable to capture the men at the top of the list of indicted war criminals, including Mr Karadzic and the chief of the Bosnian Serb army, Ratko Mladic.

Carla del Ponte said there were too many fugitives and asked Nato to make Mr Karadzic's arrest a priority.

"Failure to take him into custody ... remains a serious obstacle to any lasting peace in Bosnia and Herzegovina. Let's make justice to have peace done," she said.

But Lord Robertson said the alliance was committed to the pursuit of justice in Bosnia.

He said 17 of the 33 people now in custody for war crimes in the Hague had been apprehended by Nato troops - three of them in the last three months.

He said Nato would continue to maintain that pressure and suggested that the assassination of another indicted war criminal, the Serb warlord Arkan, in Belgrade at the weekend, could encourage others to give themselves up.

Western impotence

"It may well be that the violent death of Arkan, who was indicted by the tribunal, may encourage others to seek civilised justice in the Hague rather than waiting for some other form of arbitrary justice to be meted out to them," he said.

"There is no hiding place for those who have been indicted."

But BBC correspondent Oana Lungescu says critics of Nato argue that the alliance is afraid to take casualties or to create political upheaval in Bosnia ahead of local and general elections this year.

Jacques Klein, the head of the United Nations Mission in Bosnia, recently said that the failure to arrest Mr Karadzic demonstrated the impotence of the West in the face of evil.