An al-Qaida affiliate is claiming responsibility for Sunday's attack on a U.N. peacekeeping camp in northern Mali Sunday, killing 10 blue helmets from Chad and wounding 25 others.
U.N. Secretary-General Antonio Guterres condemned the attack and called it a possible war crime.
His spokesman said Guterres sends his "heartfelt condolences" to the government of Chad and the victims' families. He paid tribute to the "dedication and courage of the men and women serving ... at great personal risk and sacrifice."
Al-Qaida in the Islamic Maghreb is claiming responsibility. It calls the attack a reaction to Chad resuming diplomatic relations with Israel.
Guterres described Sunday's attack as "complex." Gunmen pulled up in several armed vehicles and ambushed the peacekeeping camp in Aguelhok, in northern Mali's Kidal region.
U.N. forces repelled the assailants, but not before 10 peacekeepers died and 25 were wounded.
No one has claimed responsibility for the attack. The number of assailants killed is unknown.
U.N. peacekeepers have been in Mali since 2013, not long after Islamic militants took advantage of the chaos stirred by a failed coup in the capital to seize parts of the north.
A 2015 peace agreement signed by the government and armed groups has failed to stop violence by Islamic extremists, who also stage attacks in neighboring Burkina Faso, Ivory Coast and Niger.
Guterres said such attacks "will not diminish the resolve of the United Nations to continue supporting the people and government of Mali in their efforts to build peace and stability in the country."