BEIRUT (AP) — Clashes broke out between two powerful insurgent groups in northern Syria on Tuesday, leaving up to seven people dead in the most serious fighting in months in the last major rebel stronghold in the country.

The al-Qaida-linked Hayat Tahrir al-Sham — Arabic for Levant Liberation Committee — and the Turkey-backed Nour el-Din el-Zinki group blamed each other for triggering the fighting in the northern province of Aleppo.

Nour el-Din el-Zinki is part of a 15-member coalition known as the National Liberation Front that has clashed with extremists in the past. Other factions in the NLF have been sending reinforcement to rebel-held parts of Aleppo to back their allies against al-Qaida-linked gunmen raising fears that the fighting will escalate.

According to activist collectives in northern Syria, both groups used heavy weapons, including tanks, in the fighting.

The rebel-held area is mostly in the northwestern province of Idlib that has witnessed sporadic violence since a Russia and Turkey agreed on a truce in September that averted a government offensive on the area.

Idlib has been plagued by assassinations over the past months that left scores of people dead including al-Qaida-linked fighters.

The government-controlled Syrian Central Military Media said the al-Qaida-linked fighters captured the villages of Taqad, Saadiyah and Habata. It added that fighting is ongoing in the town of Daret Azzeh.

The Levant Liberation Committee said Nour el-Din el-Zinki militants shot dead five people, including four of its fighters, last week. It added that a local court released an official with the Nour el-Din el-Zinki after questioning him leading to tensions in the area.

The clashes are the first between the two former allies since they reached a deal to end similar fighting in October.

The Britain-based Syrian Observatory for Human Rights says two civilians, including a nurse at a clinic in Daret Azzeh, were killed. SCMM said five al-Qaida-linked fighters were killed as well.