The barrage of bombardment pounded several areas in the northern provinces of Aleppo and Idlib, where government troops are waging a Russian-backed assault against rebels and jihadists.

"Regime raids hit two villages in the south of Aleppo province, killing 15 civilians," said Rami Abdel Rahman, who heads the Britain-based Syrian Observatory for Human Rights monitoring group.

In the neighbouring province of Idlib, government air strikes killed five civilians in the town of Saraqeb, the Observatory said.

That broad region is held by Hayat Tahrir al-Sham (HTS), which is dominated by Al-Qaeda's one-time affiliate in Syria.

Syrian government troops launched a ferocious offensive in late December to retake parts of Idlib and secure a key road leading from Aleppo south to the capital.

Regime forces have made key gains, recapturing the Abu Duhur military airport and dozens of nearby villages.

Since it erupted in 2011, Syria's conflict has morphed from a protest movement into a brutal and complex war that has left 340,000 people dead.

In an attempt to bring an end to the fighting, backers of opposing sides last year agreed to four "de-escalation" zones in the country.

Idlib makes up part of one zone. The other three are in Syria's south, the central province of Homs, and an area called Eastern Ghouta, an opposition enclave near Damascus.

On Thursday, three children were killed in artillery fire on Eastern Ghouta, the Observatory said.

The area has been besieged by government troops since 2013 and is home to around 400,000 people living a humanitarian nightmare.

In apparent retaliation, rebel rocket fire hit the government-controlled capital and killed three people, said state news agency SANA.