An agreement was reached on "a unilateral ceasefire by Syrian government forces in the Idlib de-escalation zone, from 6:00 am on August 31," the Russian Reconciliation Centre for Syria said in a statement.

The announcement came after Russian-backed regime forces advanced in the militant-held bastion, with monitors reporting air strikes that killed civilians including children.

In Washington, Secretary of State Mike Pompeo voiced concern over the "horrific effects" on civilians from the air strikes by Assad and his allies.

"He underscored the need for an immediate return to a ceasefire," the State Department said in a statement after Pompeo met the UN special envoy on Syria, Geir Pedersen.

The Russian statement said the ceasefire aimed "to stabilise the situation" in Idlib and urged anti-government fighters to "abandon armed provocations and join the peace process".

After months of intense bombardment, regime forces launched a ground offensive against Idlib earlier this month.

Syria's war has killed more than 370,000 people and displaced millions since it started in 2011.

Russia launched a military intervention in support of Assad in 2015, helping his forces to reclaim large parts of the country from opposition fighters and militants.