MOSCOW — The Russian army has said President Bashar al-Assad's Syrian forces had ceased fire "unilaterally" in a northwestern jihadist bastion, but a war monitor said deadly bombardment continued there.

The Syrian Observatory for Human Rights said air strikes by regime ally Russia resumed on the Idlib region late Sunday, after shelling and rocket fire by regime forces earlier in the day killed six civilians.

A months-old buffer zone deal is supposed to protect the Idlib region held by Syria's former Al-Qaeda affiliate from a massive government assault, but regime and Russian bombardment has increased since late April.

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

"However, firing targeting government forces' positions and civilians in the provinces of Hama, Latakia and Aleppo continues," it added.

The Syrian state agency SANA said earlier 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 (HTS).

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

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.  AFP