Air strikes on Syria's former Al-Qaeda affiliate in the country's northwest on Tuesday killed 26 people, including civilians, a monitoring group told AFP.

The Syrian Observatory for Human Rights said at least 10 air strikes battered the headquarters of Fateh al-Sham Front and the surrounding neighbourhood in Idlib city at dawn on Tuesday.

"Ten civilians, mostly women, are among the dead," Observatory head Rami Abdel Rahman told AFP.

"The toll could rise because some people are trapped under the rubble and unaccounted for," he added.

Abdel Rahman said the raids were likely carried out by Russian warplanes -- allied with Syria's government -- or by a US-backed air coalition.

Fateh al-Sham has come under increasing pressure in recent weeks in Idlib province, the only remaining opposition-held province in war-ravaged Syria.

Bombing raids against the group have escalated, including one US strike in January that killed more than 100 fighters at a training camp in Idlib province.

The US-led coalition has mostly focused on Fateh al-Sham's jihadist rival, the Islamic State group, but it has also hit operatives from other groups.

Rebel groups have held Idlib province since the spring of 2015, four years after conflict first broke out in March 2011.