Russia yesterday said it had "irrefutable information" Syrian rebels were planning an imminent "provocation" in Idlib province to justify Western intervention.

Moscow has long said anti-regime groups in the last major rebel bastion were planning an attack on civilians, as Russian and regime strikes on the province intensify.

"Those taking part in the staging of provocations should be fully ready by the evening of September 8," the Russian military's spokesman Igor Konashenkov said in a statement.

He said Moscow had evidence leaders of "terrorist" groups had gathered in Idlib and were being aided by the White Helmets volunteer rescue service.

In April, the US, France and Britain launched joint missile strikes on Syrian targets in response to an alleged chemical weapons attack in the town of Douma that left scores of people dead.

Russia stuck by its ally Syria and angrily insisted the Douma attack was staged by the White Helmets.

The accusation came as America's top general yesterday said he was involved in "routine dialogue" with the White House about military options should Syria ignore US warnings against using chemical weapons in an expected assault on the enclave of Idlib.

Marine General Joseph Dunford, chairman of the Joint Chiefs of Staff, said no decision had been made by the United States to employ military force in response to a future chemical attack in Syria.

Syrian President Bashar al-Assad has massed his army and allied forces on the front lines in the northwest, and Russian planes have joined his bombardment of rebels there, in a prelude to a widely expected assault despite objections from Turkey.

This week, a top US envoy said there was "lots of evidence" that chemical weapons were being prepared by Syrian government forces in Idlib.