Belgrade, Yugoslavia ? Warning that patience is running out, the American envoy monitoring war crimes in the Balkans urged Yugoslav leaders on Friday to hand over suspects to a U.N. tribunal or face sanctions that could ruin the country’s economy.

Yugoslavia could lose tens of millions of dollars in badly needed U.S. aid and American support for relations with international monetary organizations, such as the World Bank. But Yugoslav authorities have been reluctant to extradite allies of former President Slobodan Milosevic to the U.N. war crimes tribunal in The Hague, Netherlands.

Pierre-Richard Prosper, U.S. ambassador-at-large for war crimes, interrupted his official travels in Africa and came to Belgrade because of what he described “the seriousness of the situation.”

“We will have to consider seriously withdrawing our support and preventing some (other aid) coming through as well,” Prosper said after talks with Serbian Prime Minister Zoran Djindjic.

Congress gave Yugoslav authorities until March 31 to cooperate with the tribunal or else lose $120 million in U.S. assistance. Because the deadline has passed, no U.S. aid checks can be written for economically struggling Yugoslavia until Secretary of State Colin Powell certifies its compliance.

Yugoslav President Vojislav Kostunica on Thursday downplayed the U.S. sanctions threat, saying it was a hype created by his political archrival Djindjic.