A group of civilians abducted in Sudan's Darfur region on their way to a conference are still missing, the UN said on Wednesday, despite a rebel leader's order at least two days ago to free them. Ali Al-Za'tari, the United Nations chief in Sudan, said he is "extremely concerned by the continued abduction of Sudanese civilians travelling from Zalingei in Central Darfur to Nyala, South Darfur, to attend a conference on internal displacement." He called their seizure a serious crime. The case highlighted security problems faced by 1.4 million Darfuris uprooted by the region's decade-old conflict, and whose future was under discussion at the conference which ended late on Tuesday. Abdel Wahid Mohammed al-Nur, who heads a faction of the Sudan Liberation Army, told AFP on Monday night that his forces had seized the 31 internally displaced people (IDPs) in error. "One of the commanders made a big mistake," he said. "I gave (an) order to release these people, civilians, immediately." He could not be reached on Wednesday. The African Union-UN peacekeeping mission in Darfur (UNAMID) said it was escorting the displaced people to the event when they were stopped on Sunday evening "by a large unidentified armed group in military uniforms and seven jeep-mounted guns". Darfur officials said the incident was an attempt to sabotage the conference but they also blamed UNAMID for not protecting the IDPs. Central Darfur Governor Yousif Tibbin said peacekeepers showed "no resistance" to the gunmen, the official SUNA news agency reported. Darfur's top official, Eltigani Seisi, told the refugee conference that UNAMID must be "stronger", another SUNA report said. Sources in the region told AFP the peacekeepers were outnumbered, and that the incident occurred near a Sudanese government checkpoint. Critics have previously accused UNAMID of not being aggressive enough in fulfilling its core mandate of protecting civilians. The mission has defended its role and says it has helped improve security in Sudan's western region. In a statement on Wednesday, UNAMID said its local commander asked for reinforcements immediately after the IDPs were seized. The backup blue helmets were stopped on their way by Sudanese troops, "which prevented the peacekeepers from attempting to release the IDPs," the statement said. Ten years ago Nur and other rebels from black tribes began an insurrection seeking an end to what they said was the domination of Sudan's power and wealth by Arab elites. In response, government-backed Arab militia known as Janjaweed shocked the world with atrocities against ethnic minority civilians. Villages were burned to the ground. Although the worst of the violence has long past, rebel-government clashes continue while inter-Arab fighting and banditry add to the instability.