The Russian army has said President Bashar al-Assad's Syrian forces had ceased fire "unilaterally" in north western Idlib province, where the UN fears a "humanitarian catastrophe".

"From 00:00 on 18 May, Syrian armed forces unilaterally ceased fire in the Idlib de-escalation zone," the Russian military's centre for reconciliation in Syria said in a statement.

The statement came after Britain, France, the United States and eight other countries at the UN Security Council warned on 10 May of a potential humanitarian catastrophe from an all-out assault in Idlib region, in a statement opposed by Russia.

Western powers are concerned that the Russia-backed Syrian government will launch a full-scale assault, despite a deal reached with Turkey, who back rebel forces, to set up a de-escalation zone in Idlib.

The Syrian Observatory for Human Rights, however, said that shelling by the regime's forces had continued, despite the announcement by the Russian army.

Syria's war has killed more than 370,000 people and displaced millions inside Syria and abroad

The Syrian state agency Sana said rockets and mortar shells were fired at the northern part of Hama province by "terrorist groups", using the regime's terminology for jihadists and rebels.

It added that the army responded with fire that destroyed positions held by Syria's former Al-Qaeda affiliate Hayat Tahrir al-Sham.

The group controls most of Idlib province as well as parts of neighbouring Aleppo, Hama and Latakia provinces.

Syrian forces and their Russian allies have stepped up air strikes and shelling in the jihadist-controlled Idlib region since late April, raising alarm over a possible looming full-on offensive by Damascus to seize the territory.

About three million people live in Idlib, the largest area still outside the control of the Syrian government.

Syria's war has killed more than 370,000 people and displaced millions inside Syria and abroad since starting in March 2011 with a crackdown on anti-government demonstrations.