"The decision to launch the investigation came after the ICC had finished its analysis of the referral by the UN Security Council," Yves Sorokobi, spokesman for the ICC, said on Monday. "This included consultations with experts and ensuring we had met all our statutory requirements before beginning the investigations."

He added, "We will be taking all necessary steps in carrying out the investigation, including sending teams into the country and making arrangements for cooperation with all the stakeholders, both at the national and international level."

The UN Security Council referred the situation in Darfur to the ICC prosecutor on 31 March. The resolution required Sudan and all other parties to the conflict to cooperate with the court.

"We are actively seeking the cooperation and support of the Sudanese government and hope that as a member of the UN, Sudan will cooperate fully with our investigation," Sorokobi said.

On 5 April, UN Secretary-General Kofi Annan handed over to Ocampo a sealed list of 51 suspected perpetrators of atrocities in Darfur.

Sudan has not ratified the ICC's governing Rome Statute, and opposition to the referral of the situation in Darfur to The Hague-based court is widespread in Africa's largest country.

Sudanese President Umar al-Bashir has previously said his government would not hand over any citizens to be tried outside the country. He said Sudan's own judiciary was qualified and ready to try those accused of any violations in Darfur.

In April, the country's Council of Ministers declared its "total rejection" of UN Security Council Resolution 1593, calling for those accused of war crimes in Darfur to be tried before the ICC. The Sudanese council said it lacked "justice and objectivity".

"It will form part of a collective effort, complementing African Union [AU] and other initiatives to end the violence in Darfur and to promote justice," Ocampo said.

"Traditional African mechanisms can be an important tool to complement these efforts and achieve local reconciliation," he added.

Ocampo also invited the court and the AU to discuss practical arrangements that will facilitate the work of the ICC, including the possibility of conducting proceedings in the region.

Annan established the International Commission of Inquiry on Darfur in October 2004. The commission reported to the UN in January that there was reason to believe crimes against humanity and war crimes had been committed by the government, the Janjaweed militias and rebel forces in Darfur, and recommended that the situation be referred to the ICC.

In a statement issued on Monday, New York-based advocacy group Human Rights Watch (HRW) welcomed the ICC's decision to start the investigation on alleged atrocities in Darfur, saying it was a key step towards attaining justice.

"The ICC prosecutor's decision to investigate mass slaughter and rape in Darfur will start the wheels of justice turning for the victims of these atrocities," Richard Dicker, the director of HRW's International Justice Programme, said.

The war in Darfur, which began in February 2003, pits Sudanese government troops and militias – allegedly allied to the government - against rebels fighting to end what they say is the marginalisation and discrimination of the region's inhabitants by the state.