Dozens of civilians were killed by Taliban and ISIS militants, officials said on Sunday in what is a rare cooperation between the two extremists groups.

Mohammad Noor Rahmani, head of Sar-e Pul’s provincial council, said 44 of the 50 victims were believed to be civilians, with the Afghan Local Police (ALP) — a government-backed militia –also suffering casualties.

“This is not the final toll. It might change because the area is inaccessible and no telephone networks are working to get an update,” he told AFP.

The militants killed more than 50 men, women and children in the remote Sayad district of northern Sar-e Pul province on Saturday after overrunning the ALP, said local officials.

The joint Taliban-ISIS operation could increase the strain on Afghanistan’s beleaguered security forces.

“It was a joint operation by ISIS and Taliban fighters. They had recruited forces from other provinces of the country and attacked Mirzawalang village,” Zabihullah Amani, a spokesman for the provincial governor, told AFP.

The spokesman alleged that dozens of Taliban and ISIS terrorists under the command of Sher Mohammad Ghazanfar, a local Taliban commander who Amani claims pledged allegiance to ISIS, launched a coordinated attack on the area on Thursday.

“The fighters overran the area and it led to the massacre of innocent and defenseless civilians,” he said.

Amani said the fighters attacked a security outpost in the Mirza Olang area of Sayad district overnight, torching 30 houses.

“They were killed in a brutal, inhumane way,” he said.

Verifying information from poor, mountainous areas of Afghanistan made inaccessible by fighting and with patchy communications is difficult, and AFP was not able to access the village.

The Taliban had on Sunday denied any involvement in the attack, dismissing the claim as propaganda.

The Taliban and ISIS fighters have regularly clashed since the latter gained a foothold in eastern Afghanistan in 2015, as the two vie for supremacy in the war-torn country.

An Afghan security source told AFP there had been around three incidents in the past where fighters from both groups had teamed up to strike Afghan forces in certain areas.

Although the Taliban and ISIS are usually enemies, the allegiance of their forces is sometimes fluid, with fighters from both groups sometimes changing sides or cooperating with militants from other groups.

“This is not the first time that they have cooperated. There are no strict ideological distinctions between them so they build bridges when it helps them both. It’s very opportunistic,” the Afghan source said.

ISIS has been adding a sectarian twist to the Afghan conflict, with a number of deadly attacks on Shi’ites in the past year.

Last week two suicide bombers throwing grenades killed more than 33 worshippers at a mosque in Afghanistan’s western city of Herat, in an attack claimed by ISIS.

Zabihullah Mujahid, a spokesman for the Taliban, confirmed to AFP that it had captured Mirzawalang village but said it had done so alone. It also denied allegations it had killed civilians.

“It was an independent operation by our mujahideen forces. There is no cooperation with the Islamic State on the operation,” said the spokesman.