KHARTOUM (Reuters) - Gunmen ambushed U.N./African Union peacekeepers in Sudan's Darfur region and exchanged fire with them for 25 minutes in the third attack on the force in less than a week, officials said on Wednesday.

The unidentified attackers opened fire on the Pakistani armed police unit on Tuesday afternoon as it was escorting a convoy just outside Nyala, capital of south Darfur, the UNAMID mission said.

No one was injured and the attackers fled after a force of Nigerian UNAMID soldiers came to the scene to help, said force communications chief Kemal Saiki.

"It is a scandal that our forces are being targeted ... They are impartial. They take no sides," he told Reuters.

"Once again we are having to emphasise the fact that our forces are in Darfur to try and help bring peace and stability."

Saiki said he did not want to jump to any conclusions about whether the shooting was linked to two attacks on UNAMID units on Friday and Saturday in north Darfur that killed five Rwandan peacekeepers.

"There have been successive attacks. But it is difficult to jump to conclusions. This one took place in Nyala which is a different area altogether."

Twenty-two UNAMID peacekeepers have been killed in action in Darfur since the undermanned mission moved to the western region in January 2008. It has regularly been targeted by bandits and car-jackers who have continued to plague the region, despite an overall decline in the levels of military violence.

Darfur's conflict surged in 2003 when mostly non-Arab rebels took up arms against the Khartoum government, accusing it of neglecting the region. Khartoum mobilised mostly Arab militias to quell the uprising, unleashing a wave of violence that Washington calls genocide, a term Khartoum rejects.

Government officials and a former Darfur rebel group said two groups of men had been arrested in connection with Friday's and Saturday's attacks. In both cases officials said the gunmen were criminals trying to steal vehicles.