Islamist militants attacked a facility housing several aid groups in northeast Nigeria at the weekend in what the United Nations warned on Monday is an escalation in violence specifically targeting aid workers.

It was not immediately clear which militant group was responsible for Saturday’s attack in Ngala, near the border with Cameroon. A more than decade-long insurgency by Islamist groups in northeastern Nigeria has killed 36,000 people and left more than 7 million in need of humanitarian assistance.

Three witnesses told Reuters that at least 20 displaced people awaiting assistance were killed in the attack on the facility where aid workers live and provide assistance to displaced people.

A statement by the United Nations on Monday said that its five staff members who were there at the time were not harmed.

“I am shocked by the violence and intensity of this attack, which is the latest of too many incidents directly targeting humanitarian actors and the assistance we provide,” U.N. Humanitarian Coordinator Edward Kallon said.

The insurgents struck on Saturday evening, firing on people from their convoy of vehicles carrying explosives and pick-up trucks mounted with anti-aircraft guns as it entered the town, according to witnesses.

Humanitarian staff at the facility escaped before the militants overpowered Nigerian security forces guarding the compound, said Bakaka Mallam Bor, who saw the attack.

“A few minutes after, they detonated the car filled with explosives, setting the hub ablaze and burning the humanitarians’ vehicles,” Bor said.

Kallon said aid workers are increasingly targeted by militant groups, noting that 12 were killed in 2019, double the previous year, and two remain in captivity.

On Dec. 22, unknown militants killed at least 10 people in a convoy in northern Nigeria in an attack that sources told Reuters targeted Christians and those associated with international aid groups.

The Islamist insurgency began with the Boko Haram group in 2009, but an offshoot – Islamic State in West Africa Province (ISWAP) – has in the last two years been the dominant faction.