A suicide car bomb attack on an army training camp in Yemen's second city of Aden killed at least 60 people on Monday, medical sources said.A security official told AFP that the attacker drove his vehicle into a gathering of new recruits at the camp in northern Aden. The assault killed 60 people and wounded 29 others, medical sources from the three hospitals where the victims were taken told AFP.Security officials had provided an earlier toll of 11 dead. The port city, the temporary base of Yemen's Gulf-backed government, has seen a wave of bombings and shootings targeting officials and security forces.Attacks in Aden are often claimed by extremists from either al Qaeda or the Islamic State group, which have both taken advantage of the chaos in Yemen to make gains in southern and southeastern regions. But no group claimed immediate responsibility for Monday's attack.Yemeni authorities have trained hundreds of soldiers in Aden over the past two months to as part of operations to retake neighbouring southern provinces from extremists.Earlier this month, Yemeni government forces backed by a Saudi-led coalition entered Abyan's provincial capital Zinjibar. Troops retook other towns across Abyan but have been met by fierce resistance in key al Qaeda stronghold, al Mahfid, a town which lies further east, security sources said.The militants are still present in areas surrounding the recaptured towns and control large parts of the neighbouring Shabwa province, the sources say. The Arab coalition which backs the Yemeni government against Iran-backed rebels has also been providing troops with air cover throughout their war against the extremists.The coalition intervened in Yemen in March last year and has helped government troops push the rebels out of Aden and four other southern provinces. But authorities have been struggling to secure these provinces.More than 6,600 people, mostly civilians, have been killed in Yemen since March 2015 and more than 80 per cent of the population has been left in need of humanitarian aid, according to the UN.