Infighting between two of the most powerful factions in Syria's opposition-held Idlib province has killed at least 14 people in the past 24 hours, a monitor said Wednesday.

The fighting involves the jihadist Hayat Tahrir al-Sham (HTS), led by a former affiliate, against one-time ally Ahrar al-Sham, a powerful Islamist rebel group.

The Syrian Observatory for Human Rights said fighters from the two sides were engaged in clashes across the province in northwest Syria that killed 11 fighters and three civilians in the last 24 hours.

"These are the most violent and widespread clashes that have taken place between Ahrar al-Sham and Tahrir al-Sham," Observatory director Rami Abdel Rahman told AFP.

"The clashes are ongoing across all of the province, with territory changing hands... It's an existential battle," he added.

An AFP correspondent in the province also reported fighting in several areas, particularly in the town of Sarmada and around the towns of Saraqeb and Al-Dana.

He said both sides had set up multiple checkpoints inside and around provincial capital Idlib city.

The two groups have clashed before, despite having previously formed the backbone of the alliance that captured most of Idlib in early 2015.

The latest conflict arises partly out of a dispute over Ahrar al-Sham's desire to fly the flag of the Syrian uprising in Idlib city, the Observatory and AFP's correspondent said.

Idlib city was only the second provincial capital to fall from government control, and the province is one of the last remaining strongholds of the rebels.

More than 330,000 people have been killed in Syria since the conflict began with anti-government protests in March 2011.