Secretary of State Colin Powell spoke by telephone Thursday with Serbia's new Prime Minister Vojislav Kostunica, a day after he formally took office as head of a minority government that will rely on support from Serb nationalists. The State Department said U.S. relations with the Kostunica government will depend on its actions, including support for the Balkans war crimes tribunal.

Secretary Powell knows Mr. Kostunica well from his tenure as the former president of Yugoslavia. And officials here said they had a candid conversation about the future shape of relations between the United States and the new federation of Serbia and Montenegro.

Mr. Kostunica took office Wednesday as the Serbian prime minister, capping a long process of forming a government after badly-divided elections in December.

He is considered a moderate nationalist though his minority government will depend for its survival on votes of the Serbian Socialist Party, the hardline movement of his predecessor as Yugoslav president, Slobodan Milosevic, who is now on trial at the Balkans war crimes tribunal in the Hague.

At a news briefing, State Department spokesman Richard Boucher said the Bush administration is interested in working with the new Serbian government, but whether it can depends on what it does on such issues as political reform, the rule of law, and cooperation with the tribunal.

He said there were low expectations given the past history of Mr. Kostunica's governing partners but said U.S. officials are prepared to be pleasantly surprised.

"I don't think I can comment on the makeup of the government, so much as to say that it really depends on what they do," he said. "We know the history of some of these parties, particularly the Socialists and its association with Milosevic. I know that's led a lot of political commentators to tell us not to expect very much. But I think we'll have to see what the government does on important issues like reform, like the rule of law, and like cooperation with the tribunal."

Mr. Boucher said it is a matter not only of policy for the Bush administration but of law as well, since Mr. Powell is required under an act of Congress to report by the end of this month whether Serbian cooperation with the Hague tribunal is sufficient to merit continued U.S. aid.

At stake is about $100 million in bilateral aid and international loans aimed at helping post-war reconstruction in Serbia.

Mr. Kostunica is a longtime opponent of the international tribunal and says Serbian war crimes suspects should be tried at home. He has said that war crimes would not be his government's top priority, given simmering social tensions and a troubled economy.