The International Criminal Court said Monday that it had begun a formal investigation of suspected war crimes in Sudan's Darfur region, where tens of thousands of people have died since a rebel uprising began in early 2003.

"The investigation will be impartial and independent, focusing on the individuals who bear the greatest criminal responsibility for crimes committed in Darfur," the court said in a statement. It did not name any suspects.

Human rights groups welcomed the announcement, saying it would bring hope for justice to victims of killings, rape and displacement, and urged the Sudanese government to support the investigation.

The government rejected any investigation by the court that might lead to a war crimes trial. However, Majzoub Khalifa, head of the government's Darfur talks team, said the international court and others were welcome to send observers to trials in Sudanese courts.

"If they want to observe what is going on . . . they are welcome," Khalifa said. But "if they want to start trials of the Sudanese, this is not acceptable." The Sudanese government says it has already arrested members of the military and security forces for trial.

Najeeb Kheir Abdul Wahab, Sudan's state minister for foreign affairs, said after the court announcement that "the priority of the international community should be to secure a cease-fire in which people could trust for the rendering of justice."

Richard Goldstone, who served as the first prosecutor on the U.N. war crimes tribunal for the former Yugoslavia, said Sudan would have to cooperate eventually, as Yugoslavia did when it handed over former president Slobodan Milosevic. "With most of the African Union against them on this, it may be very difficult for them to resist," he told BBC television.

The U.N. Security Council voted in March to refer the situation in Darfur to the International Criminal Court, the world's first permanent global criminal court, established in 2002 to try cases of genocide and major human rights violations.

Sudanese rebels welcomed the Security Council referral and said they would hand over any of their members indicted by the court.

"The wheels of justice are finally beginning to turn on behalf of the people of Darfur who have been victims of mass slaughter, widespread rape and forced displacement," said Richard Dicker, counsel for New York-based Human Rights Watch.

The referral, the first to the court by the Security Council, was made possible when the United States abstained from the vote after winning guarantees that U.S. citizens in Sudan would be exempt from prosecution by the court, which it opposes.

In April, U.N. Secretary General Kofi Annan gave the court a sealed list of 51 men suspected of slaughter, rape and pillage in Darfur. They have not been named but are said to include top Sudanese government and army officials, militia leaders and rebel and foreign army commanders.

The Darfur conflict broke out when rebels took up arms against the Arab-dominated government on grounds of discrimination against non-Arabs in the arid region. Sudan's government is accused of retaliating by arming local Arab militias, who rebels say have burned villages and killed and raped civilians. The government denies the charges.

Hunger, violence and disease have killed about 180,000 people in Darfur since the fighting erupted; more than 2 million have fled their homes, creating a humanitarian emergency. The United States has called the violence genocide.

The 53-nation African Union has deployed about 2,300 troops to monitor a shaky cease-fire in Darfur, but sporadic violence continues. Efforts to peacefully and permanently resolve the crisis have been stalled for six months, but talks between the government and the rebels, mediated by the African Union, are expected to resume by June 10.

The International Criminal Court opened its first investigations in 2004, into crimes in Democratic Republic of Congo and Uganda. Based in The Hague, it has yet to issue its first indictment or arrest warrant. Its chief prosecutor, Luis Moreno-Ocampo, says he expects to prosecute cases this year.

The court is supported by 139 countries -- not including Sudan -- but is fiercely opposed by the United States, on grounds it might pursue politically motivated prosecutions of Americans.