KHARTOUM, Sudan  — A gunfight broke out between security forces on Thursday in the biggest city of Sudan's strife-torn Darfur region, officials and witnesses said, killing one aid worker and prompting authorities to impose a curfew.

Clashes between the army, rebels and rival tribes have surged in the vast and mostly lawless region in recent months, but had until now been confined to rural areas.

Residents said heavy gunfire could be heard for hours near the security headquarters in Nyala, the capital of South Darfur state and the second-biggest city in Sudan. Men dressed in military uniform were seen exchanging fire with people inside the security compound.

One aid worker was killed during the fighting, a spokesman for the international peacekeeping force UNAMID said, adding that details were unclear.

State authorities blamed "actions between the security forces", according to a statement, which gave no details. The government imposed a night curfew in Nyala.

"People are panicking and shops are closed," another resident told Reuters by phone as gunfire resounded in the background.

The United Nations said it had canceled a regular flight from Khartoum to Nyala for security reasons, and UN staff in the city were moved to a bunker. By nightfall they were still waiting to be evacuated to a safer UN base, a UN source said.

In separate violence, three Nigerian peacekeepers were wounded when unknown assailants ambushed them with rocket-propelled grenades on their way to Nyala on Wednesday, the UNAMID spokesman said.

Law and order have broken down in most parts of Darfur since mainly African tribes took up arms in 2003 against Sudan's Arab-led government, which they accuse of discriminating against them. Khartoum denies this.

Violence is down from its peak in 2004-05, but has picked up again this year, involving the army, rebels and Arab tribes, many of which were armed by the government early in the conflict and are now fighting among themselves over resources and land.

In April, a group of reserve policemen staged a brief mutiny in El Geneina in West Darfur state.

Events in Darfur are difficult to verify because Sudan severely restricts access to foreign journalists.

The International Criminal Court has indicted President Omar Hassan al-Bashir and other Sudanese officials for masterminding war crimes in Darfur. Sudan has dismissed the charges as political campaign against the African country.