Darfur rebels have clashed with Sudanese government forces in the northern outskirts of the capital, Khartoum, but the government says the attack on the city has been defeated.
The clash was the first time major fighting from the Darfur conflict has spilled into the Sudanese capital.
The government said it beat back the rebels Saturday after several hours of fighting in the suburb of Omdurman, across the Nile River from downtown Khartoum. However, the rebels from the Justice and Equality Movement, JEM, say some of its fighters have taken up positions in and around the city.
Sudan's interior ministry says a curfew is now in effect in the capital.
The Sudanese government accused neighboring Chad of supporting the rebels. Chad denied the claim and denounced the rebel attack.
In Washington, a spokesman for the National Security Council, Gordon Johndroe, said the United States is concerned about the situation and 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 urged a return to dialogue between Sudan's government and Darfur rebels.
The Sudanese army increased security in the capital Friday, saying an attack from JEM rebels may be imminent. It accused JEM and its leader, Khalil Ibrahim, of planning attacks as part of a publicity stunt.
Five years of fighting in the Darfur region involving rebels, the Sudanese government and government-backed militias has killed more than 200,000 people and displaced about 2 1/2 million others.
Some information for this report provided by AFP, Reuters and AP.