Sudanese officials are urging residents in the capital of Khartoum to remain indoors as security forces search for Darfur rebels who clashed with government forces Saturday.

Sudan authorities indefinitely extended a curfew in Khartoum Sunday and said they believe some rebels are still in the city.

Authorities also showed a picture of the leader of the rebel Justice and Equality Movement, Khalil Ibrahim, on state television and urged residents to call a telephone hotline if they see him. Authorities said they believe Ibrahim personally led the assault on Khartoum and is still in the area - perhaps in the neighboring city of Omdurman.

Also Sunday, Sudanese President Omar Hassan al-Bashir accused Chad of supporting the rebels and said the country has cut diplomatic ties with its neighbor. Chad denies it supported the rebels and has denounced the rebel attack.

Sudan said government forces beat back the rebels on Saturday after several hours of fighting in the northern outskirts of Khartoum. The clashes marked the first time the Darfur conflict has spilled into the Sudanese capital.

In Washington, a spokesman for the National Security Council, Gordon Johndroe, said the United States is concerned about the situation. He urged both sides to cease hostilities.

U.N. Secretary-General Ban Ki-moon condemned the attack and called for an end to the fighting.

Southern Sudan's leader, Salva Kiir, also condemned the assault, and he urged a return to dialogue between Sudan's government and Darfur rebels.

Some information for this report was provided by AFP, and Reuters.