Gunmen killed at least eight Chadian UN peacekeepers in an attack Sunday on one of their bases in northern Mali, multiple sources reported.
The UN spokesman in Mali condemned what he described as a "cowardly" attack and called for a "robust" response.
"According to a new toll, still provisional, at least eight peacekeepers have been killed," a source close to the MINUSMA force said.
A Malian security source said about 20 people were injured in the attack.
They struck early Sunday at the Aguelhok base 200 kilometres (125 miles) north of Kidal and towards the border with Algeria, the source close to MINUSMA added.
A diplomat in northern Mali told AFP that several of the attackers had also been killed.
Mahamat Saleh Annadif, the UN's special representative for Mali, condemned what he called a "vile and criminal" attack.
"Peacekeepers of the MINUSMA force at Aguelhok fought off a sophisticated attack by assailants who arrived on several armed vehicles," he said in a statement.
The attack "illustrates the determination of the terrorists to sow chaos.
"It demands a robust, immediate and concerted response from all forces to destroy the peril of terrorism in the Sahel."
Mali Islamist Violence
It was one of the deadliest attacks to strike UN forces in Mali.
An attack at the same base last April killed two peacekeepers and left several others wounded.
The deadliest strike was in October 2014, when nine troops of a Nigerian contingent were killed in the northeast.
Some 13,000 peacekeepers are deployed in Mali as part of a UN mission.
It was established after Islamist militias seized the north of the country in 2012. They were pushed back by French troops in 2013.
A peace agreement signed in 2015 by the Bamako government and armed groups was aimed at restoring stability to Mali.
But the accord has failed to stop violence by Islamist militants, who have also staged attacks in neighbouring Burkina Faso and Niger.
Earlier this month, both France and the United States criticised the authorities in Mali for their failure to stem the worsening violence.
On January 16, France threatened to push for more targeted sanctions to be imposed on Mali after hearing a UN official report on worsening violence in the West African country.
Washington renewed its warning that it would push for changes to the peacekeeping mission in Mali, possibly a major drawdown, if there was no progress.
In August, a panel of experts said in a report to the UN Security Council that inter-communal conflicts in the region were exacerbating existing tensions from clashes between jihadist groups and international and Malian forces.
On Sunday, France's Defence Minister Florence Parly told French radio that the G5 Sahel anti-jihadist force in the region was resuming its operations.
They were suspended after an attack on their headquarters in mid-2018. The G5 force comprises contingents from Mali, Mauritania, Niger, Burkina Faso and Chad.