It was the second day of bombardment on the jihadist and rebel-held province and adjacent areas, after key powerbrokers in Tehran failed to reach a deal to avert a government assault.

The Syrian Observatory for Human Rights, a Britain-based monitoring group, said heavy attacks resumed on the northwestern region near the Turkish border from around midday.

"Regime helicopters dropped more than 60 barrel bombs on the village of Hobait in Idlib's southern countryside," Observatory head Rami Abdel Rahman said.

The raids killed at least two children and wounded six other people, he said.

In the neighbouring province of Hama, Russian jets carried out more than 10 strikes on rebel positions in the village of Al-Latamneh, he said.

The raids wounded five rebels and knocked the village's underground hospital out of action, just a day after strikes damaged a similar health facility in Idlib's southern town of Hass.

The strikes eased off later Sunday, the Observatory said.

Some of the heaviest bombardment in weeks hit Idlib and nearby areas on Saturday, killing at least nine civilians, according to the Observatory.

More than half of Idlib is held by Hayat Tahrir al-Sham (HTS), an alliance led by Syria's former Al-Qaeda affiliate, while most of the rest is held by rival rebels. The regime controls a southeastern chunk.

HTS and rebels are also present in adjacent areas of the neighbouring provinces of Aleppo, Hama and Latakia.

Hundreds of families have fled Idlib's southeastern areas since Saturday, when Russian and regime strikes on the region were the most violent in a month, the Observatory said.

The United Nations has warned that any military campaign in Idlib could push up to 800,000 people to flee their homes.

The leaders of regime allies Russia and Iran met with the president of rebel backer Turkey in Tehran on Friday but they failed to reach an agreement to avoid a military assault.

The rebel-held region of Idlib and adjacent areas are home to almost three million people, half of whom have been displaced from other areas in the country, according to the UN.

Regime troops have for weeks been massing around Idlib, after President Bashar al-Assad's regime retook control of other areas of the country earlier this year.

More than 350,000 people have been killed and millions displaced since Syria's civil war started in 2011 with the brutal repression of anti-Assad protests.