Four United Nations peacekeepers have been killed and 15 others injured after the convoy they were traveling in struck a landmine in northern Mali.

The contingent of peacekeepers from Chad was struck near the embattled town of Kidal Tuesday. At least six of those hurt were seriously wounded.

The U.N. secretary general, Ban Ki-moon, condemned the attack, the latest in a series of incidents against U.N. personnel and contractors in recent weeks, including one that wounded nine Chadian peacekeepers August 29.

The United States called on the government of Mali to immediately investigate the incident and hold those responsible to account.

Northern Mali fell under control of Tuareg separatists and then al-Qaida-linked Islamic extremists following a 2012 military coup.  A French-led intervention last year scattered the extremists, but some remain active and there have been continued bursts of violence.

The government of Mali is in talks with Tuareg and Arab groups.  Al-Qaida is not participating in the discussions.