The African Union has expressed "deep regret" that two rebel groups in Darfur failed to meet the deadline to sign a peace deal for the troubled Sudanese region.

Leader of Darfur's Justice and Equality Movement Khalil Ibrahim

AU Commission Chairman Alpha Oumar Konare released a statement Thursday expressing disappointment that the Justice and Equality Movement and a Sudanese Liberation Army faction did not sign the deal.

Also Thursday, 62 Nobel Prize winners sent a letter to the leaders of several world bodies calling for the deployment of a U.N. peacekeeping force to Darfur. They also urged President Bush to designate a special envoy to work with Sudan's government and rebel factions in Darfur.

In related news, the White House Thursday announced the Republic of Congo's president Denis Sassou-Nguesso is to visit President Bush June 5 to discuss peace efforts in Sudan.

Mr. Sassou-Nguesso heads the African Union.

Last month, Sudan's government and the main Darfur rebel group, the Sudan Liberation Movement reached a peace deal. It is aimed at ending more than three years of conflict in Darfur that has killed at least 180,000 people.

The deal calls for a more equitable distribution of power and wealth, the disarming of pro-government Janjaweed militias, and a referendum on Darfur's future.

Two million people have been displaced by the fighting in Darfur involving the rebel groups, the Arab-dominated Sudanese government and the Janjaweed militias.