Al-Qaida-linked terrorists carried out one of the deadliest attacks on a United Nations peacekeeping mission in months, killing 10 peacekeepers and wounding at least 25 in northern Mali, the U.N. said Sunday.

U.N. Secretary-General Antonio Guterres strongly condemned what he described as a "complex attack" on the U.N. peace mission's camp in Aguelhok, near Kidal.

All of the peacekeepers killed in the assault were from Chad, Guterres' spokesman said in a statement. He said the attack may constitute a war crime.

Peacekeepers "responded robustly and a number of assailants were killed," the statement said. Residents said the attackers in the Sunday morning assault arrived in motorcycles and cars.

The 15,000-strong peacekeeping mission in Mali, established in 2013, is one of the U.N.'s deadliest. The West African nation is under threat from a number of extremist groups affiliated with al-Qaida and Daesh terror group, and attacks have moved from the arid north to more populated central Mali.

The U.N. mission is one of several military efforts to fight the terrorists, alongside Malian forces, France's largest overseas military operation and a recently formed five-nation regional counterterror force.

Peacekeepers from Chad, a strong contributor to regional security efforts, have suffered from the repeated assaults in Mali. As of the end of 2018, 51 Chadian peacekeepers had died while serving the U.N. mission.