A senior Russian official has denounced what he called the "two-faced policy" of the US, saying it was to blame for the death of a Russian general, Valery Asapov, in Syria, according to an RIA news agency report.

The Russian defence ministry has said Asapov was killed in shelling by the Islamic State of Iraq and the Levant (ISIL) group near Deir Az Zor.

"The death of the Russian commander is the price, the bloody price for the two-faced American policy in Syria," Sergei Ryabkov, Russia's deputy foreign minister, said on Monday, according to RIA.

Separately, Russia denied carrying out an air strike that a US-backed Syrian force said killed one of its fighters and injured several others.

The Syrian Democratic Forces (SDF) said a Russian air strike on Monday killed one of its fighters and injured two others in a gas field in eastern Syria that they had recently captured from ISIL, also known as ISIS.

Major-General Igor Konashenkov, a Russian defence ministry spokesman, said in a statement that Russia is continuing to target ISIL in the area, and that its surveillance does not show SDF forces fighting ISIL.

Konashenkov said neither the SDF nor the US-led coalition has approached it about the reported attack.

On September 16, the SDF said a Russian air strike wounded six of its fighters, but Russia denied carrying out the attack.

Civilians killed in Idlib

Elsewhere in Syria, Russian air strikes on Monday killed at least 37 civilians, including 12 children, in northwestern Idlib province, a Britain-based monitor said.

The Syrian Observatory for Human Rights (SOHR) said it was the highest civilian death toll in Idlib since the region was designated in May as one of Syria's "de-escalation" zones under an accord between government allies Russia and Iran with Turkey.

"The air raids struck several locations and villages in the district of Jisr al-Shughur, leaving 37 dead among civilians, including 12 children," SOHR's Rami Abdel Rahman told AFP news agency, updating an earlier toll of 27 dead.

After several months of calm, Idlib has for the past two weeks been the target of heavy air strikes by President Bashar al-Assad's government and its Russian allies.

The air strikes follow an assault in neighbouring Hama province also covered by the de-escalation accord for Syria's war, which has cost more than 330,000 lives since 2011.

ISIL has no fighters in Idlib.

Syria's exiled opposition blamed Russia and the Assad government for what it called the "criminal" bombing, according to a statement.

The Istanbul-based National Coalition said hospitals, medical installations, schools and civil defence facilities were hit in what it branded as "war crimes".