The United States Thursday called on allies to join Washington in pressing Sudan to accept deployment of United Nations peacekeepers in Darfur. The mandate for the current African Union observer mission in the troubled western Sudanese region expires at the end of the month.

Officials here are concerned that Darfur may be headed for a new round of severe violence if the peacekeeping issue is not settled, and they are calling on allied governments to match U.S. diplomatic pressure on Khartoum to allow deployment of a U.N. force.

Sudanese authorities had agreed in principle to permit the upgrade of the African Union mission into a full-fledged U.N. peacekeeping force before the U.S.-mediated Darfur peace accord was signed in Nigeria last May.

But they have since been resisting the plan, which would triple the size of the current 7,000 member international presence, and provide needed mobility and logistical support.

At a news briefing, State Department Spokesman Sean McCormack dismissed an assertion Thursday by Sudanese President Omar al-Bashir that proponents of the U.N. force have a hidden agenda, and want to re-colonize Sudan.

McCormack said the history of dealing with the Sudanese government is that it is responsive over time to concerted diplomatic pressure, and he appealed to other countries to match U.S. efforts at persuasion with Khartoum.

"This doesn't move as quickly as we would like," he said.  "Certainly not. We have been pressing hard on this. We would certainly hope that the rest of the world would join us in pressing as hard on this issue as we have. I think it is incumbent upon other member states of the United Nations to match the effort the United States has made on this issue."

The Khartoum government has been threatening a major military offensive in Darfur, and the main Darfur rebel group has warned it is prepared to resume fighting if African Union troops leave as scheduled at month's end.

Darfur rebels took up arms in 2003 against the Sudanese government, which supported Arab militiamen in a scorched-earth campaign in the region. Since then, war and disease have killed more than 200,000 people and displaced another 2 million.

Secretary of State Condoleezza Rice met Monday with Sudanese Foreign Minister Lam Akol, who carried a message to the Bush administration from President Bashir.

U.S. officials said the message expressed a desire for improved bilateral relations but did not break any new ground on the peacekeeping issue.

Secretary Rice said later she told the foreign minister in no uncertain terms that improved ties were not on the agenda unless Sudan acted responsibly and agreed to accept, and facilitate, a U.N. force.

Bilateral relations are sharply limited because of Sudan's continued presence on the State Department's list of state sponsors of terrorism.