First responders have accused Syria's military and its ally Russia of a "massacre", after nine civilians were reportedly killed in strikes on camps in the opposition-held north-west.

The White Helmets said rockets loaded with cluster bombs hit camps for the displaced in Idlib province on Sunday.

Three children and a woman were among the dead, the organisation added.

A Russian military official said Syrian warplanes struck a jihadist group's drone facility and training camp.

The UN said it was deeply concerned by the escalation of hostilities.

About 4.1 million people dependent on humanitarian aid, most of them women and children, are trapped in the last stronghold of the jihadist and Turkish-backed rebel groups opposed to President Bashar al-Assad, after 11 years of a civil war that has left half a million dead.

The region regularly sees armed clashes, air strikes and shelling despite a ceasefire brokered by Russia and Turkey in 2020 to halt a government offensive.

The UN has documented the deaths of at least 121 civilians there since the beginning of this year alone.

The White Helmets, whose volunteer first responders operate in opposition-held areas, alleged in a statement that the Syrian and Russian militaries carried out "a deliberate, calculated, and premeditated attack" on three camps for internally displaced people near the villages of Kafr Jales, Morin and Wadi Hajj Khaled, west of Idlib city, on Sunday morning.

The strikes caused fires and destroyed the tents and homes of hundreds of displaced families.

One of those killed was the infant son of another first responder, Hussein Bakir, according to the White Helmets.

The organisation also accused Russia of a "media disinformation campaign" over the strikes. It cited a statement on Saturday by a top Russian military official in Syria, Oleg Yegorov, who accused the White Helmets of collaborating with al-Nusra Front - a precursor to the jihadist alliance that dominates Idlib, Hayat Tahrir al-Sham - to stage attacks on camps in Idlib.

There was no immediate response to those allegations from Russia, but Mr Yegorov said on Sunday that the Syrian military struck al-Nusra Front facilities in retaliation for a drone attack that killed five Syrian soldiers.

On Monday, three UN humanitarian officials demanded an immediate investigation into the attacks on the camps and implored combatants "to act with restraint, and to prioritise measures safeguarding the safety and security of civilians and civilian infrastructure".