Militants overran an army base in northeastern Nigeria, killing more than 30 soldiers before pulling back in the face of air strikes, soldiers and a resident said.

The attackers were believed to belong to the regional offshoot of Islamic State. They hit the base in Mainok town in northeast Borno state on Sunday afternoon, three soldiers and the resident told Reuters.

Rising insecurity across Nigeria has killed scores of soldiers and civilians this year. Just over a month ago, about 30 soldiers were killed in four attacks by Islamist militants in northeast Nigeria.

In a statement issued Monday evening, a military spokesman said that one officer and six soldiers were killed and five were wounded in the attack. The statement said troops had killed scores of militants and were now in full control of the area.

The sources told Reuters that 33 soldiers were killed in Sunday’s attack. The militants wore military camouflage and arrived in around 16 gun trucks and six mine-resistant military vehicles, one of the soldiers said. After several hours, they captured the base and soldiers called in airstrikes.

More soldiers were killed when militants ambushed reinforcements sent to help, the soldier sources said. A resident said the attackers also set ablaze the town’s police headquarters.

“I saw them while fighting with soldiers,” resident Ba Umar Abba Tuja told Reuters. “When the fighter jet started hovering in the air, the (militants) fled to the community and hid in the primary school.”

Tuja said the militants left around midnight.

One soldier said air strikes had hit army troops due to their similar dress as the militants. The Air Force, in a post on Twitter, said it was investigating the incident.

Mainok is roughly 55 km (30 miles) from Maiduguri, the capital of Borno state. An Islamist insurgency has plagued northeast Nigeria for more than a decade, killing more than 30,000 people and displacing at least 2 million.

Islamic State West Africa Province, which broke away from Boko Haram several years ago, now stages its own attacks on soldiers and civilians in the region.

Reuters