BEIRUT (Reuters) - Syrian Islamist fighters have killed scores of insurgents they took captive during clashes, a rebel official and the SITE Intelligence Group said.

An offshoot of the Jund al-Aqsa group killed roughly 150-200 members of rebel factions and of another Islamist alliance in the village of Khan Sheikhoun in southern Idlib province, they said.

They had been arrested and held captive for six days during clashes that broke out in the area, in the increasingly bloody infighting between different militant groups.

Dozens of those executed were members of a Free Syrian Army (FSA) faction, Jaish al-Nasr.

Abdul Hakim al-Rahmon, head of Jaish al-Nasr’s political wing, confirmed that 70 fighters from the group were executed eight days ago, vowing to attack in response.

He said more than 160 FSA fighters were killed in total, plus another 43 from Tahrir al-Sham - which includes the former al Qaeda branch in Syria, Jabhat Fateh al-Sham - who had been killed immediately after the militants stormed a court run by Tahrir al Sham in the same area.

“They were all liquidated at the same time,” he said.

The U.S.-based security monitoring service SITE reported on Wednesday, citing a pro-al Qaeda social media outlet, that a total of 150 members of rebel factions had been killed.

They included more than 70 from Jaish al-Nasr, and fighters from the Tahrir al-Sham alliance.

Jund al-Aqsa is seen as ideologically close to Islamic State, al Qaeda’s main jihadist rival.

Fighting between Jund al-Aqsa and Tahrir al-Sham has flared in the past week, in clashes that war monitors say have killed dozens.

Those clashes have added to the complexity of insurgent infighting in the west of the country.

Both Tahrir al-Sham and Jund al-Aqsa are also fighting against FSA rebel factions who have been foreign-backed. Jihadist groups attacked the FSA for sending delegates to peace talks in Kazakhstan last month.

Many of those FSA groups are now fighting under the banner of the more moderate but powerful Islamist Ahrar al-Sham.