"We will not support any form of partition," Undersecretary of State Nicholas Burns said at a news conference. And, he said, "the great majority of countries would not stand for that."
Kosovo, which has declared independence with strong U.S. support and over the anger of Belgrade, has a majority ethnic Albanian population. Many Serbs view Kosovo as the cradle of their culture and Orthodox Christian faith.
There are some 100,000 Serbs living in Kosovo, most of them in the north, with several enclaves in southeastern Kosovo. They have ignored Kosovo's declaration and threatened to set up their own institutions.
In Belgrade on Wednesday, Serbia's Prime Minister Vojislav Kostunica demanded that the United States reverse its recognition of Kosovo. Russia also is strongly opposed to an independent Kosovo.
"As a nation and a state, we will put up resistance every day until the United States is convinced that the rule of international law must be re-established in the Balkans and the illegal declaration of the fake state (of Kosovo) is annulled," Kostunica said.
Burns said most countries support an independent Kosovo. In contrast, Serbia's foreign minister, Vuk Jeremic, writing in The New York Times on Wednesday, said it will not be recognized by a vast majority of U.N. nations. He called the unilateral declaration of independence an illegal act and a "historical injustice" to Serbia.
Burns, asked about the article, fired back that the Serbs were forgetting the history of the 1990s when the Serbs were accused of ethnic cleansing of ethnic Albanians in the Balkans.
"It is hypocritical for the Serb foreign minister to go to our newspapers and our media and act as if nothing happened in 1998 and 1999, when those terrible injustices were suffered by the Kosovar Albanian Muslim population," Burns said.
Serbia tried to force one million ethnic Albanians out of the country, and because of that history the United Nations took Kosovo away from Serbia, Burns said. "We have not forgotten," he said.
Rioters, protesting U.S. support for Kosovo independence, set fire to the U.S. Embassy in Belgrade last week. Burns said the United States would hold Serbia responsible for "what happens in the streets of Belgrade."