Russian President Vladimir Putin, who backs Syria's government, met the leaders of fellow regime ally Iran and rebel supporter Turkey to determine the fate of the northwestern zone on the Turkish border.

Government forces have been massing around Idlib for weeks ahead of an expected offensive on the province, which is held by jihadists led by Al-Qaeda's former Syrian affiliate and rival Turkish-backed rebels.

On Friday morning, Russian air raids targeted rebel positions in the southwest of the province, the Syrian Observatory for Human Rights said.

Among them were positions of the jihadist Hayat Tahrir al-Sham (HTS) alliance, as well as of the hardline Ahrar al-Sham group, the Britain-based monitor said.

They destroyed one Ahrar al-Sham post, killing four of its fighters and wounding 14 others in the area of Hobait, it said.

An AFP stringer saw rescue workers working with their bare hands to retrieve a victim from rubble blocking the entrance of what appeared to be a cave inside a sandstone rock face.

They carried away the limp body of a man covered in pale dust, as diggers worked nearby to clear the debris.

A shepherd was also killed and four other people wounded in the bombardment, the Observatory said, although it was not immediately clear if they were fighters or civilians.

"The aim was to destroy rebel fortifications," said Observatory head Rami Abdel Rahman.

HTS controls more than half of Idlib province, while other rebels, including Ahrar al-Sham, hold most of the rest.

Regime troops are present in a southeastern chunk of the province, but observers say a planned offensive could target other peripheral areas of the rebel-held zone.

On Friday, Putin said that he, Iranian President Hassan Rouhani and Turkish leader Recep Tayyip Erdogan "discussed concrete measures regarding a phased stabilisation" for Idlib.

But a joint statement released after the talks gave few details.

Aid groups have warned that any military offensive in Idlib could spark one of the worst humanitarian disasters of Syria's seven-year civil war.

Almost three million people live in Idlib and adjacent rebel-held areas, half of whom have already been displaced from other parts of the country, the United Nations says.

More than 350,000 people have been killed and millions displaced since the war erupted in 2011 with the brutal repression of anti-government protests.