BEIRUT (Reuters) - At least nine members of Islamic State were killed during infighting in northwestern Syria after some of them tried to flee over the Turkish border, a group monitoring the conflict said on Monday.

The fighters clashed on Saturday near the town of al-Bab, 30 km (20 miles) south of the Turkish frontier, the Britain-based Syrian Observatory for Human Rights said. Five of the escapees and four of those trying to prevent them were killed.

Those trying to escape included one Tunisian and nine European fighters, the Observatory's Rami Abdulrahman said, adding it was not clear exactly which members of that group had been killed.

It is not the first time that Islamic State, which controls tracts of Syria and Iraq, has killed its own members. The Observatory reported in December that the group had killed more than 120 of its fighters in two months, most of them foreigners trying to return home.

The fighting on Saturday erupted when 10 Islamic State combatants broke out of a prison run by the group. They had tried to escape Syria previously and had been jailed, Abdulrahman said, citing sources on the ground.

The five escapees who were not killed were recaptured by the group, the Observatory said.

Thousands of foreigners from across the world have joined the group, an offshoot of al Qaeda, which last year declared a caliphate in territory it controls.

(Reporting by Sylvia Westall; Editing by Robin Pomeroy)