DOUALA, Cameroon (Reuters) - Gunmen in military fatigues and masks have killed 22 people in a village at the heart of a separatist insurgency in western Cameroon, shooting women and children and burning others in their homes, the United Nations said on Monday.

Cameroon’s army has since 2017 been fighting English-speaking militias seeking to form a breakaway state called Ambazonia amid the cocoa farms and forests of west Cameroon. As fighting has intensified, so have abuses by both sides, witnesses and rights groups say.

The fighting is the gravest threat to stability in the oil- and cocoa-producing country since President Paul Biya took power nearly 40 years ago.

It was not yet clear who was responsible for Friday’s attack in Ntumbo in the northwest region of Cameroon near the Nigerian border. In a statement, separatists blamed the army. In its own the statement, the army denied wrongdoing.

Survivors “were extremely shocked and traumatized. People just left their houses and left everything behind,” said James Nunan, an official with the U.N. humanitarian coordination agency OCHA that conducted interviews with witnesses and survivors.

Nunan said that 14 of the dead were children, some of whom were under five. At least 600 people fled, he said.

The type of attack, with people being burned alive and shot, echoes other raids that witnesses told Reuters were committed by the military. The army has denied involvement in those raids.

The government said on Monday that its soldiers were on a reconnaissance mission in Ntumbo when they were attacked. The fighting that followed caused several fuel containers to explode and set nearby houses ablaze, it said, killing five civilians.

“In light of the methodically and professionally cross-checked information, it is simply an accident, collateral damage of the operations to restore security in the region,” the government said in a statement.

The separatists said at least 35 civilians were killed in what they called a “violation of the human rights of the Ambazonian people.”

Conflict between Cameroon’s army and English-speaking militias began after the government cracked down violently on peaceful protesters by lawyers and teachers in 2016 complaining of being marginalized by the French-speaking majority.

Violence spiked again in the run-up to parliamentary elections on Feb. 9, rights groups said, including the burning of houses.

Nearly 8,000 Cameroon refugees fled to eastern and southern Nigeria in the first two weeks of February, the United Nations refugee agency said, adding to the more than half a million people who have already left.