Nearly 70 fighters from two formerly allied Islamist militant groups have been killed during infighting over the past 24 hours in northern Syria, a monitor said on Tuesday.

The clashes between former Al-Qaeda affiliate Fateh al-Sham and the hardline Islamist militant Jund al-Aqsa faction erupted on Monday morning, after tensions over influence in the northwestern province of Idlib.

The fighting reflects the growing strained relations between different factions in Idlib province that once fought alongside either other against President Bashar al-Assad's forces.

The Britain-based Syrian Observatory for Human Rights said the clashes erupted after Jund al-Aqsa carried out a suicide bomb attack against a Fateh al-Sham headquarters in Idlib, killing nine people.

The toll has now risen to 69 dead from both sides in heavy clashes as well as executions, with the fighting spreading to the neighbouring province of Hama.

"There are battles between warlords, it's a war for influence," said Observatory director Rami Abdel Rahman.

Jund al-Aqsa is reviled by most rebels in the region, and is designed a "terrorist group" by Washington.

Despite that, in October Fateh al-Sham announced it had taken Jund al-Aqsa under its wing, although clashes between the two groups erupted shortly afterwards.

In January, Fateh al-Sham also battled other rebel groups in Idlib during 10 days of clashes that killed dozens of fighters.

Idlib province is held almost entirely by opposition factions, and was captured by an alliance of fighters dubbed the Army of Conquest, led by Fateh al-Sham.

More than 310,000 people have been killed in Syria since the conflict began in March 2011 with anti-government protests.

The war has become a complex multi-front conflict, drawing in Islamist militant groups and international armies.