Armed men killed at least 160 people in an attack on a village in northern Burkina Faso, the country's worst attack in recent years, officials say.
Homes and the local market were burned during the raid on Solhan in the early hours of Saturday morning.
No group has said it was behind the violence, but Islamist attacks are increasingly common in the country, especially in border regions.
The UN chief said he was "outraged" by the incident.
António Guterres "strongly condemns the heinous attack and underscores the urgent need for the international community to redouble support to member states in the fight against violent extremism and its unacceptable human toll," his spokesperson said.
On Sunday, a total of 160 bodies were recovered from what local officials in Solhan described as three mass graves, AFP news agency reported.
"It's the local people themselves who have started exhuming the bodies and burying them after transporting them," an unnamed local source said, according to AFP.
The number of people killed in Solhan has increased from earlier reports of about 132 deaths, and there are concerns that the death toll may rise further.
Following the news of Saturday's attack, the Burkinabe President Roch Kabore declared three days of national mourning, writing in a tweet that "we must stand united against the forces of evil".
He described the attack as "barbaric" and said that security forces were trying to track down the perpetrators.
In another attack on Friday night, 14 people were reported to have been killed in the village of Tadaryat, about 150km (93 miles) to the north of Solhan.
Last month, 30 people died in an attack in the east of Burkina Faso.
The country is facing a deepening security crisis, like many of its neighbours, as armed groups carry out raids and kidnappings across much of the region.
In May, the Burkinabe army launched a large-scale operation in response to a resurgence of militant attacks. Despite this, the security forces are struggling to prevent the violence that has forced more than a million people from their homes over the past two years.
Africa's semi-arid Sahel region has been hit by an insurgency since militants captured large parts of northern Mali in 2012 and 2013.
French forces have been supporting troops from Mali, Chad, Mauritania, Niger and Burkina Faso to fight the militants.
But this week France halted co-operation with Mali over the recent coup there.