Heavily armed militants carried out an “extremely violent” attack on a vital aid facility housing United Nations workers in northeast Nigeria’s Borno state, the U.N. said on Monday, January 20.

No aid workers were harmed in the assault, but a military source said that one soldier and four assailants died in the ensuing gunfight.

The humanitarian hub in Ngala “was the direct target of a complex assault by heavily armed non-state armed groups operatives” on Saturday, the U.N. Office for the Coordination of Humanitarian Affairs said in a release.

Humanitarian and military sources said militants in trucks fitted with machine guns stormed the hub which is near a camp for displaced people, AFP reported. An aid worker said the insurgents fired anti-aircraft machine guns and rocket-propelled grenades as they engaged soldiers in battle.

“Protective security measures deployed at the humanitarian hub prevented any harm to the staff who was in the facility,” OCHA said.

“The soldiers managed to evacuate the aid staff to their base close by while fighting continued,” the aid worker said.

At least 20 displaced people awaiting assistance were killed, Reuters reported witnesses as saying.

A Nigerian military officer said that the attackers “had also abandoned a vehicle laden with explosives intended for a suicide attack near the humanitarian hub.”

The U.N. said that an “entire section of the facility was burned down as well as one of the few vehicles U.N. agencies rely on for movement and aid delivery.”

It published images that showed a burned building and a destroyed vehicle.

In a Sunday statement, Islamic State said fighters from its West Africa Province affiliate “attacked a garrison of the Nigerian apostate army, in the town of Gamboru Ngala” on Saturday. It said clashes led to the death and wounding of “a number of” soldiers.

Ngala is just outside Gamboru in eastern Borno state, across the border from Fotokol in Cameroon. On January 6, an explosion at a crowded market on the bridge that connects Gamboru and Fotokol killed at least nine people, although some reports said dozens were killed.

Saturday’s attack is the latest to target aid workers trying to tackle the vast humanitarian crisis caused by the decade-long jihadist insurgency in northeast Nigeria.

The U.N. said it “was outraged by the extremely violent attack on this key humanitarian facility where five United Nations staff were staying at the time of the incident.”

Aid workers are providing assistance to more than 55,000 people in Ngala. More than 10,000 people arrived there in 2019.

U.N. humanitarian coordinator for northeast Nigeria Edward Kallon said he was “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.”

Twelve aid workers were killed in northeast Nigeria in 2019 as the conflict has become increasingly perilous for those trying to deliver humanitarian assistance to the hundreds of thousands of displaced. Two female humanitarian workers are still being held by after being kidnapped.

The jihadist group known as Boko Haram began its bloody insurgency in northeastern Nigeria in 2009, but it has since spread into neighboring Niger, Chad and Cameroon, prompting a regional military response.

Boko Haram split into two factions in mid-2016. One, which is also known as JAS, is headed by long-time leader Abubakar Shekau and is notorious for suicide bombings and indiscriminate killings of civilians. Shekau pledged allegiance to then-ISIS leader Abu Bakr Al-Baghdadi in March 2015, but ISIS central only gives formal backing to the other faction, which it calls Islamic State West Africa Province.

ISWAP largely focuses on attacking military and government targets. Its main area of operations is the Lake Chad area of Nigeria, Chad and Niger, and to a lesser extent Cameroon, but the group has intensified attacks on military locations in west of Borno state capital Maiduguri in recent months.

Gamboru, a trading hub, has been rocked by violence since August 2014 when Boko Haram seized the town along with nearby Ngala. After months of fierce battles, Nigerian troops retook both towns in September 2015 with the help of Chadian forces.

But jihadist fighters continue to launch sporadic attacks on both sides of the border, ambushing troops, carrying out suicide bomb attacks and raiding refugee camps. Both Boko Haram and ISWAP have claimed or been blamed for attacks in the area.

The U.S. assesses that Boko Haram and ISWAP are responsible for more than 35,000 deaths since 2011. More than two million people have been displaced by the conflict, sparking a dire humanitarian crisis in the region.

With reporting from AFP