A helicopter of the U.N.-African peacekeeping mission in Darfur was shot at during a routine mission and forced to return to its airfield, a spokesman for the force said Monday.

The incident came as special U.S. envoy to Sudan, Richard Williamson, was visiting a different part of the arid, wartorn region, some 300 kilometers away.

UNAMID spokesman Noureddine Mezni said the helicopter came under fire half an hour into its mission, damaging the radio system and the rear of the aircraft, according to an initial inspection.

There were no casualties and the helicopter made it back to El Geneina, the capital of West Darfur province.

Mezni said it was not immediately clear who attacked the helicopter.

A Sudanese security official in West Darfur, however, said the aircraft was hit by rebels from the Justice and Equality Movement, who operate in the region. Speaking on condition of anonymity because he was not authorized to speak to the media, the official said the aircraft had "deviated" from its course.

There was no way to independently confirm the report and there was no immediate comment from the rebel group.

JEM has emerged as the most effective rebel group in Darfur fighting government troops.

The joint U.N.-African Union peacekeeping mission in the Darfur region has often come under attack from armed gunmen in the region. An ambush on its forces in July killed seven peacekeepers.

U.S. envoy Williamson, meanwhile, is visiting El Fasher, in the North Darfur province, where he met with UNAMID senior officials in the mission's headquarters .

Maj. Gen. Emmanuelle Karenzi, the deputy commander of the mission, said the meeting with Williamson lasted for 40 minutes, during which he was briefed on ways to speed up and enhance the force deployment. He didn't elaborate.

"(Williamson) expressed his support for the mission and concern over the fact deployment is so slow," Karenzi said in a telephone interview.

The force took over peacekeeping duties in Darfur in January from a beleaguered AU force to try to stem the violence, but eight months after its deployment it is still operating with about one-third of its authorized 26,000.

It also lacks sufficient combat and rescue helicopters.

When renewing its mission this month, the U.N. Security Council endorsed an ambitious plan to increase the force to 80 percent of its authorized level by the end of the year.

Fighting erupted in Darfur in 2003 when ethnic African rebels took up arms against the Arab-dominated central government, accusing it of discrimination. The Sudanese government is accused of responding by unleashing the janjaweed militias in a reign of terror on Darfur's population.

The Khartoum government denies the allegations. Up to 300,000 people have been killed and more than 2.5 million displaced since the fighting began.

Al-Bashir was accused last month by the International Criminal Court prosecutor of directing a genocide in Darfur. The ICC is reviewing a request to issue an arrest warrant for al-Bashir.