Syrian regime air strikes killed five civilians including two children Sunday in a northwestern bastion of the opposition, a war monitor said.

A Turkish observation post in Syria's Idlib region was attacked with mortar fire and shelling from an area controlled by Syrian government forces, causing damage but no casualties, the Turkish Defence Ministry said on Sunday.

Clashes on the edge of a militant bastion in northwestern Syria have killed 35 fighters in 24 hours, a monitor said Monday, after regime bombardment on the region devastated health services.

Attacks by Syrian regime forces and their Russian allies killed 12 civilians in the country's northwest on Friday in the latest violation of an eight-month-old truce, a war monitor said.

Fighting in northwestern Syria has displaced nearly 140,000 people since February, the UN said on Wednesday, as the regime and its ally Russia have stepped up their bombardment.

A New Zealand nurse believed abducted with two drivers by Islamic State militants in Syria in 2013 may still be alive, the International Committee of the Red Cross has revealed for the first time in an appeal for news of her whereabouts.

Twin explosions in the bustling centre of the rebel-held northwestern Syrian city of Idlib on Monday killed at least 15 people and injured scores, medics and witnesses said.

Turkey is doing what is required to maintain peace and prevent violations in Syria's northern Idlib, the last major rebel stronghold in Syria, Turkey's foreign minister said on Monday.

Turkey loaded tanks and armoured vehicles onto trucks on Saturday and dispatched the convoy to the Turkish province of Hatay on the Syrian border, the Demiroren news agency said.

Turkey sent a convoy of commando units and armoured vehicles to the Turkish province of Hatay on the Syrian border, state-owned Anadolu news agency said on Friday.

Two activists critical of both the regime and jihadists were gunned down Friday in Syria's last major rebel bastion in the northwest of the country, their radio station and a monitor said.

Some militants have withdrawn from the demilitarisation zone in Idlib following a deal between Russia and Turkey on the northwestern Syrian region, a US official said on Wednesday.