Russian warplanes battered the Syrian opposition stronghold of Idlib on Tuesday (September 4th) for the first time in three weeks, as expectations mount of a government offensive in the northwestern province, AFP reported.

Children were among at least nine civilians killed in the strikes, the Syrian Observatory for Human Rights said.

Regime ally Moscow and opposition backer Ankara have held several rounds of talks aimed at averting an assault, but government troops have been massing near the opposition zone.

"Russian warplanes resumed bombing Idlib province after a 22-day pause," said Observatory head Rami Abdel Rahman.

He said at least nine civilians, including five children from the same family, were killed in the raids, while 10 people were wounded.

The strikes targeted 24 areas and came a day after opposition units in Idlib "hit regime positions in neighbouring Latakia province, which killed three pro-regime fighters", Abdel Rahman said.

Tuesday's bombardment hit several areas held by the Tahrir al-Sham extremist alliance, among them the large town of Jisr al-Shughur, but also areas held by rival Turkish-backed opposition fighters, including the town of Ariha.

In the town of Muhambal, southwest of the provincial capital city Idlib, firefighters battled a huge blaze at a petrol station which was hit by a strike.

Warplanes continued to fly over the area throughout the afternoon, an AFP correspondent said.

Warnings of 'human catastrophe'

US President Donald Trump on Monday warned Syria against launching an attack on Idlib with the help of Russia and Iran, saying the offensive could trigger a "human tragedy".

The US envoy for Syria was in Ankara on Tuesday to discuss the latest developments in the war-torn country with Turkish officials.

Turkish Defence Minister Hulusi Akar discussed the Syrian war with James Jeffrey, the US special representative for Syria engagement, the Turkish defence ministry said in a statement, but gave no further details about the content of the meeting.

Turkish Foreign Minister Mevlut Cavusoglu had said on Monday that the two would discuss Idlib.

Cavusoglu warned Russia last month that seeking a military solution in Idlib would cause a "catastrophe" and trigger a new flow of refugees across Turkey's borders.

Meanwhile, the UN's Syria peace envoy called Tuesday on the Russian and Turkish presidents to urgently speak to each other to help avert a "bloodbath" in Idlib.

Staffan de Mistura appealed to "President Putin and to President Erdogan, ... to make a telephone call", even before they are set to meet with their Iranian counterpart in Tehran on Friday.

The presidents of Turkey, Russia and Iran are due to meet in Tehran on Friday for a tripartite summit likely to focus on Idlib.