EL FASHER, Sudan — African peacekeepers searched for more than 20 members of their force still missing and feared wandering the wilderness of Darfur on Monday after rebels overran their base in an unprecedented attack that stunned the international force.

The attack Sunday on the Haskanita base in Darfur, a western region of Sudan, illustrated the region’s chaos ahead of peace talks later this month, and the dangers that could face a bigger United Nations force due to start deploying in coming weeks.

A Nigerian army spokesman said the deadliest attack on peacekeepers in Darfur could prompt his country — which leads the current AU mission — to reconsider its commitment to trying to calm the continent’s hot spots. That also could prove catastrophic for the bigger, joint AU-U.N. force — largely African — that is due to be deployed in Darfur later this month and assume responsibility for the area on Dec. 31.

Darfur rebels have grown increasingly hostile to the struggling 7,000-member African Union force, saying it favors the government and has failed to protect Darfur civilians. The AU denies any favoritism and has often complained its forces are overstretched and undersupplied.

In Sunday’s attack, around 1,000 well-armed rebel gunmen overwhelmed the small Haskanita base of about 150 troops before dawn after hours of fighting, during which some of the African soldiers ran of ammunition. The battle killed at least 10 peacekeepers in the deadliest attack on the AU force since it deployed in June 2004. The rebels retained control of the base until early Sunday, when Sudanese government troops arrived and routed them.

The rebels looted ammunition and armored vehicles, and it took Sudanese troops to chase them away as AU troops evacuated.

The scene was chaotic at the base Sunday afternoon. AU troops with their belongings were ferried out by helicopter to safety, while Sudanese troops stood in combat positions nearby.

Twenty-three peacekeepers were unaccounted for after the attack and were believed to be wandering around the barren area near Haskanita, AU officials said.

The underfunded AU force is soon to be merged into the more powerful hybrid U.N. force. The first units of the 26,000-strong joint AU-U.N. force are due to be deployed in October.