BEIRUT (Reuters) – Around 150 Turkey-backed Syrian rebels arrived in Idlib province on Thursday to reinforce rebel group Ahrar al-Sham’s escalating conflict with a rival Islamist insurgent group formerly known as the Nusra Front, rebel sources told Reuters.

Ahrar al-Sham and Hayat Tahrir al-Sham, a faction led by the formerly al Qaeda-affiliated group earlier known as the Nusra Front, are vying for dominance in Idlib, the only Syrian province that is entirely under rebel control.

Tensions have been building between the two mainly over ideological differences between Islamist militant and more nationalist-leaning armed factions and fighting broke out this week.

A rebel source told Reuters a group of around 150 fighters from the Turkey-backed Euphrates Shield operation had entered Syria at the Bab al-Hawa crossing from Turkey to support Ahrar al-Sham in its fight against the Nusra Front.

Bab al-Hawa, a major commercial and civilian gateway into rebel-held Idlib, is controlled by Ahrar al-Sham.

Turkey launched operation Euphrates Shield last year to support certain Syrian rebel groups in order to drive Islamic State militants and the Kurdish YPG militia away from its border.