UNITED NATIONS — The request to arrest Sudan’s president, Omar al-Bashir, on charges that include genocide may force a rethinking of international policies on Darfur, but it also poses major challenges for the United Nations and for America.

The staunchest supporters of the International Criminal Court hailed yesterday’s decision by the Hague-based prosecutor, Luis Moreno-Ocampo, to request 10 indictments against Mr. Bashir in the first ICC case against a head of state. But at the United Nations many officials have expressed concerns — not only about the future of U.N. peacekeeping missions in Sudan, but also about what the new judicial landscape means for international justice.

America, specifically, is ambivalent. It has long refused to join the ICC for fear that politicization of an ill-defined international judicial system would quickly turn against top American officials. But in 2005 America declined to veto a U.N. Security Council resolution that referred the prosecution of atrocities in Darfur to the Hague-based court. The decision was seen by diplomats as a tacit blessing of the ICC.

There is a “very interesting shift in the position of the United States in the case of international justice,” France’s U.N. ambassador, Jean-Maurice Ripert, told The New York Sun yesterday in a phone interview. Along with France’s foreign minister, Bernard Kouchner, Mr. Ripert was an early advocate of the Rome Statute, shaping much of the language of the treaty that was signed 10 years ago this week and became the legal basis of the ICC.

As Mr. Ocampo comes to the United Nations to present his case against Mr. Bashir to the press tomorrow, the Security Council is expected to discuss the renewal of the peacekeeping force in Darfur, which is commanded jointly by the United Nations and the African Union. The current mandate of the force, known as Unamid, expires July 31. U.N. planners have become concerned about the viability of the force, which depends on Khartoum’s cooperation, in the aftermath of the ICC charges against Mr. Bashir.

Sudanese officials say Mr. Bashir has no intention of cooperating with the ICC. “The limit is the sky” in response to “this insult to the entire Sudanese nation,” Khartoum’s U.N. ambassador, Abdalmahmood Abdalhaleem Mohamad, told reporters yesterday. His country would stick to its international obligations, he said, but he did not exclude severing ties with the Darfur peacekeeping force, known as UNAMID, in response to the ICC charges.

U.N. officials said yesterday that most of the 20,000 UNAMID troops have withdrawn to their headquarters in the Darfur town of El Fasher, out of concerns about retaliation. Nonessential personnel were also withdrawn from major Western diplomatic posts in Sudan.

A three-judge panel is expected to rule whether to issue an arrest warrant against Mr. Bashir, though their decision is not expected for three months. Mr. Ocampo’s charges against him included counts of genocide, war crimes, and crimes against humanity. “The evidence shows that the commission of such crimes on such a scale, over a period of five years, and throughout Darfur, has required the sustained mobilization of the entire Sudanese state apparatus,” Mr. Ocampo told reporters.

“I have never seen him jubilant and strong like what I detected from the call,” Mr. Mohamad said yesterday after phoning Mr. Bashir. “Don’t be surprised” if he attends the annual gathering of General Assembly members in New York in September, he added.

Khartoum’s confidence may derive from doubts about the ICC’s enforcement abilities. If an arrest warrant is issued, and if consequently he attempts to land in New York, it is doubtful that America, which does not recognize the court’s jurisdiction, would arrest Mr. Bashir, according to the International Justice Director of Human Rights Watch, Richard Dicker. “If Bashir comes to New York, he will enjoy the same protection that Yasser Arafat and Fidel Castro enjoyed” when attending U.N. gatherings under diplomatic immunities, Mr. Dicker said.

America “has been at the forefront of holding those responsible for genocide accountable,” a State Department spokesman, Sean McCormack, said yesterday. But, he added, “We are not part of the ICC. We are not signatories of the Treaty of Rome that created the ICC.”

Some diplomats speculated yesterday that one of Khartoum’s allies might ask the Security Council to invoke a provision in the Rome Statute that allows deferring ICC cases for a year if they present a major security threat.

Mr. Ripert said yesterday that France and the European Union strongly support the independence of the court. But, he hinted, Mr. Bashir may yet escape an arrest warrant. “If he wishes to cooperate with the ICC, it is not too late,” he said, calling on Mr. Bashir to cooperate with previous ICC warrants against a mid-level Khartoum official and a militia leader of the government-allied Janjaweed fighters.