The International Criminal Court announced Monday it will launch a formal investigation into suspected war crimes in Sudan's Darfur region.

The investigation is expected to be the largest handled by the court since it was established in June 2002.

The decision follows a vote by the United Nations Security Council in March to let the ICC try people accused of committing war crimes in Sudan's Darfur region, the first case the council has referred to the ICC. The U.S., which opposes the ICC because it fears it could be used to launch politically motivated prosecutions against Americans, had agreed not to use its veto power on the Security Council.

UN Secretary General Kofi Annan has given the ICC a list of 51 people, including top Sudanese government and army officials, militia leaders and army commanders, suspected of slaughter and rape in Darfur.

The ICC is not expected to get help from Sudan, which has insisted on prosecuting any suspects itself. The UN says Sudan has done little to disarm the Arab militia.

Critics say the Sudanese government has been supporting the Janjaweed, Arab militias accused of attacking Darfur's black residents and carrying out a brutal campaign to drive out the local population. The government denies the charge.

Tens of thousands of Darfur residents have died and more than 2 million have been displaced from their homes.

Human-rights groups and other observers – including former U.S. secretary of state Colin Powell – have condemned the violence as genocide.