Bamako - A driver was killed in an ambush on a peacekeeping supply convoy in northern Mali on Monday, the United Nations said, the third deadly assault on the mission in less than a week.The UN's Minusma peacekeeping mission said in a statement the civilian contractors were targeted at 11:30 (11:30 GMT), around 30km west of Gao, the main city in the restive desert north."Initial reports indicate that at least one driver was killed, his truck was later set on fire," the statement said.Minusma chief Mongi Hamdi voiced outrage over "these heinous crimes involving innocent civilians". "We will adjust our security arrangements so that such crimes are not repeated. Minusma cannot tolerate this," he was quoted as saying.He called for the perpetrators to be brought to justice and urged Malian military and police to tighten security in the area. It was not immediately clear how the victim was killed, but the attack came after two drivers were shot dead on Friday as a Minusma supply convoy was ambushed nearby.The assailants stopped the convoy some 15 kilometres from Gao and "coldly killed two of the drivers", according to Minusma.The killings come amid continued violence by jihadist groups that took control of northern Mali before being routed by a French-led international intervention that began in 2013.A suicide bomber struck a UN barracks in northern Mali's Ansongo last Wednesday, killing two civilians and wounding nine peacekeepers from Niger in an attack later claimed by an al-Qaeda-linked group.The al-Qaeda-linked group of notorious one-eyed Algerian militant Mokhtar Belmokhtar has claimed responsibility for that attack.Earlier this month a civilian was killed in Gao by a rocket attack by presumed jihadists.Jihadists have also hit farther south, including in Mali's capital Bamako.At least 35 peacekeepers have been killed since Minusma was deployed in July 2013 - one of the highest tolls for a UN peace mission - and more than 140 wounded.