The bombing raids hit a string of towns and villages in the northwestern province of Idlib, which is almost entirely controlled by various jihadist and hardline rebels.

In the town of Taftanaz the strikes killed 10 civilians, including four children, said the Britain-based Syrian Observatory for Human Rights.

Observatory head Rami Abdel Rahman said strikes had also hit near a children's hospital, putting it out of service.

Five other civilians were killed in raids that hit other towns.

The air strikes came a day after jihadists from Al-Qaeda's former Syrian affiliate launched an attack on Fuaa and Kafraya, two villages held by the regime but cut off by hardline forces.

Late Saturday, Hayat Tahrir al-Sham (HTS) and allied fighters shelled Fuaa and Kafraya heavily and clashed with local fighters.

"This is the fiercest attack in around three years," Abdel Rahman said, adding that Sunday's deadly air strikes were in response to the jihadist attack.

Six Syrian pro-regime fighters and at least three from HTS were killed in the fighting, which continued on Sunday.

Syrian state news agency SANA also reported Saturday's attack, and said local fighters were able to push back the jihadists.

Fuaa and Kafraya are the only two places in Syria currently designated as besieged by the United Nations after the government recaptured the Yarmuk Palestinian camp in southern Damascus.

The villages are home to an estimated 8,100 people, most of them Shiite Muslims.

They came under siege in 2015, when rebels ousted regime forces from a vast majority of Idlib province.

Since then, the mainstream opposition's influence has dwindled as jihadists solidified their grip on the province, and regime forces have recaptured an eastern sliver of Idlib.

HTS and its allies control around 60 percent of the province, its local rivals hold about a third, and the regime controls about 10 percent, according to the Observatory.

Now, even the Islamic State group is conducting hit-and-run attacks in Idlib against its HTS rival.

Last week, clashes between HTS and an IS sleeper cell made up of Iraqi jihadists left more than two dozen fighters dead, mostly from IS.

In retaliation, the IS cell executed five HTS fighters it had abducted in Idlib, the Observatory said. HTS also killed six IS members it had captured in the clashes.

Siege tactics have been used throughout Syria's seven-year conflict, mostly by the government.

Troops have employed the tactic alongside heavy bombing to cut off food and medicine to rebel-held areas, then coerce people to agree to leave in population transfers.

Idlib, which lies on the border with Turkey, has seen its population balloon to around two million people in recent years as fighters and civilians evacuated from other opposition areas are dumped there.

It was designated last year as a de-escalation zone, but violence has been creeping up again.