Fighting is flaring across Sudan's embattled western Darfur region, the African Union warned Sunday. The AU reports at least 22 people have died in attacks by armed militias in west Darfur. The north Darfur capital of El Fasher is on high alert following militia attacks on Saturday. Noel King reports for VOA from Khartoum.

At least 22 people have died in western Darfur, following an attack on a commercial vehicle by unidentified armed militias, the African Union said Sunday.

An eight-member AU investigation team which traveled to Sirba village in west Darfur to inspect the site has been detained in a police station by village residents, the AU said. Sirba village is a stronghold of the rebel National Redemption Front, a coalition of rebel factions which have refused to sign on to the Darfur peace agreement.

African Union spokesman Nourredine Mezni appealed for the release of the AU observers, who he said have been prevented from boarding their helicopter to leave the area.

"We are trying our best to release our people who are now kept in Sirba area," he said. "I'm appealing to all the parties to remind them that we are in Darfur to help them to restore peace and stability. We are not part of the conflict. Any attack on our personnel or property will be seen as a violation of the cease-fire and serious measures will be taken against those who are harming our people."

In a separate incident, at least two people were killed and 11 wounded following an clash between armed militias and civilians in the north Darfur capital of El Fasher late Saturday.

The African Union says it is investigating the incident.

Former Darfur rebels of the Sudan Liberation Movement, the only rebel faction to sign the May peace agreement with the Sudanese government, called the attack a blatant violation of the accord.

Saif Haroun, a spokesman for the SLM told VOA on Sunday the marketplace in El Fasher was attacked and shops looted by scores of men who arrived in trucks.

Haroun said the SLM is frustrated by the continued attacks on civilians and will defend against further attacks.

"The situation in El Fasher is so bad right now. We are going to work on the agreement, which has been signed by the government," said Haroun. "But if the security situation is still going in this way, the SLM is going to have an option. If the international community or the African Union or the government does not take their responsibility, we are going to defend our people in Darfur."

Observers fear the SLM may make good on recent threats that it will return to war if violence against civilians continues.

The United Nations says it was forced to evacuate 134 UN and non-governmental staff from El Fasher earlier this week, following attacks by militias. Several aid agencies have evacuated staff as violence rises across Darfur.

The Sudanese government this week blamed the rising tide of violence on rebels, who have refused to sign on to the Darfur peace agreement.

U.S. Special envoy Andrew Natsios arrived in Khartoum on Saturday. He is expected to visit the Darfur region during his trip.

The United States has called the conflict, in which tens of thousands have died and some two million more have been displaced, genocide.