BEIRUT, Feb 16 (Reuters) - Syrian Islamist fighters have executed scores of insurgents in the west of the country in an increasingly bloody battle between different militant groups, the SITE Intelligence Group said.

An offshoot of the Jund al-Aqsa group killed more than 150 members of rebel factions in the village of Khan Sheikhoun in southern Idlib province, the U.S.-based monitoring service reported on Wednesday.

Dozens of those executed were members of a Free Syrian Army (FSA) faction, it said, citing a pro-al Qaeda social media outlet.

The rest included members of the Tahrir al-Sham alliance, which includes the former al Qaeda branch in Syria, Jabhat Fateh al-Sham.

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.

(Reporting by John Davison; editing by John Stonestreet)