In the toughest warning since the Dayton accord was signed 18 months ago, ending the three-and-a-half year war which claimed an estimated 200,000 lives, Carlos Westendorp said he would start by trying to persuade the authorities in the two entities within Bosnia, and in Serbia and in Croatia, to hand over indicted war criminals as required by Dayton.
So far, of 75 indicted war criminals, the only people brought before the court have been a Bosnian Croat who gave himself up and a Bosnian Serb, Dusan Tadic, who was seized by German police. Another seven await trial.
The most wanted men - Bosnian Serb war leaders Radovan Karadzic and Ratko Mladic - are still at large in Bosnian Serb territory. The Nato-led Stabilisation Force - S-For - is under instructions to detain wanted men if it chances upon them, but it never does.
Of the former warring factions, only the Bosnian Muslims have shown any inclination to hand over wanted men.
Mr Westendorp, a former Spanish foreign minister, takes over from former Swedish prime minister Carl Bildt on 20 June, and yesterday indicated he was going to take a much tougher line. "The second option is pressure," he said in Madrid yesterday. "Those who fulfil the commitments will have our support and those who do not, will have no support at all".
US Secretary of State Madeleine Albright went further, demanding that Croatia and Serbia to stop "stonewalling" .
Mr Westendorp said the resolution of the war criminals issue was crucial to maintaining the peace in Bosnia. Dayton was supposed to create a model for a multi-ethnic country, but so far the boundaries between the former warring factions have solidified.
"We cannot deviate from Dayton. We must not", Mr Westendorp said. "Coexistence among the communities is not possible unless this issue of war criminals is solved".
Immediately after the accord, the peace implementation force took the view that peace was more urgent than justice, and that attempts to seize indicted war criminals would be resisted. But after 18 months, most of the wanted men remain free, and progress towards implementing Dayton has been far too slow.
The US is determined to move its troops out before the deadline of July 1998. But Britain's new Foreign Secretary,Robin Cook, is among many Europeans who favour keeping forces there for a longer period.