KHARTOUM  - Dozens of suspected Janjaweed militiamen have ambushed a UN convoy in Sudan's Darfur region, killing seven peacekeepers and wounding 22 in the deadliest attack suffered by the beleaguered mission. Armed with heavy weaponry and travelling in 40 vehicles, the attackers ambushed the police and army convoy on Tuesday at Um Hakibah in North Darfur State, southwest of the peacekeeping headquarters in El Fasher. Seven peacekeepers were killed and 22 were wounded, seven critically, UN spokeswoman Michele Montas told reporters in New York. UN Secretary-General Ban Ki-moon, who was flying back from the G8 summit in Japan, "condemns in the strongest possible terms this unacceptable act of extreme violence against AU-UN peacekeepers in Darfur," she said. The peacekeeping force known as UNAMID, which is severely under-equipped and under-manned, has suffered a string of attacks since it assumed control from an African Union force in Darfur, gripped by escalating insecurity and banditry. Officials from UNAMID in Sudan had earlier confirmed one dead and six missing from the attack, adding that the wounded peacekeepers were receiving medical treatment in El Fasher. The peacekeepers were attacked while returning from following up allegations by the Minni Minnawi faction of the Sudan Liberation Army, which signed a 2006 Darfur peace deal with the government, that two former rebels had been killed. Two UN officials in Sudan said that Janjaweed - state-backed Arab militia - were suspected of carrying out the attack. UNAMID deputy force commander Karenzi Karake went to assess the situation following the ambush. Last month, Arab militiamen briefly abducted and assaulted a UN official and three colleagues held at gunpoint during a standoff near the main UNAMID military base in El Geneina, the capital of West Darfur. In late May, dozens of heavily armed men on horseback ambushed a UNAMID patrol in Darfur and seized weapons from Nigerian troops near El Geneina, and in a separate incident a Ugandan policeman was found murdered in North Darfur. Details of the latest attack emerged as visiting British Foreign Minister David Miliband said it was difficult to see future optimism in Darfur, which has been riven by conflict for more than five years. Until security is provided and some kind of political process is underway, it would be wrong to be anything other than "extremely cautious about the prospects, because of the scale of devastation that has already happened," he said. "I think it's very difficult for those of us who live in wealthy countries and lead comfortable lives to talk about optimism when there is such a humanitarian crisis that exists in Darfur," he told reporters. "I think that the scale of the humanitarian tragedy, never mind the killing, means that it's very important that political leaders, especially from outside the country are not glib about the situation there." The World Food Programme, the largest UN humanitarian agency, has cut rations by half because banditry has made the roads increasingly dangerous. Since UNAMID took over from a small African Union force on December 31, only 7,600 troops and 1,500 police are on the ground - barely a third of the projected total of 19,500 soldiers and 6,500 policemen. The force lacks the air transport and cover desperately needed to support troops across terrain with limited roads, as well as transport vehicles. The United Nations says up to 300,000 people have died from the combined effects of war, famine and disease and more than 2.2 million fled their homes since the conflict broke out in February 2003. Sudan says 10,000 have been killed. The conflict broke out when ethnic minority rebels took up arms against Khartoum and state-backed Arab militias often called Janjaweed, fighting for resources and power in one of the most remote and deprived places on earth.