United Nations human rights chief Michelle Bachelet has called for an immediate end to hostilities in Idlib, saying that the latest Syrian ceasefire has "yet again failed to protect civilians".

"It is deeply distressing that civilians are still being killed on a daily basis in missile strikes from both the air and ground," Bachelet said in a statement on Friday addressing the ceasefire, which was due to be implemented nearly a week ago.

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Some 350,000 Syrians, mostly women and children, have fled a renewed Russian-backed offensive in the opposition-held Idlib province since early December and have sought shelter in border areas near Turkey, the UN said in a report on Thursday.

Russian jets and Syrian artillery have pounded towns and villages in a renewed assault backed by pro-Iranian militias and aimed at clearing the opposition.

"This latest wave of displacement compounds an already dire humanitarian situation on the ground in Idlib," David Swanson, Amman-based UN regional spokesman for Syria, told Reuters News Agency.

Russian and Syrian jets resumed bombing of civilian areas in the opposition enclave two days after a ceasefire agreed between Turkey and Russia formally took effect on Sunday.

Russia's Ministry of Defence on Thursday denied reports it had bombed civilian targets in Idlib province, saying there had been no military flights since a ceasefire was introduced on January 9, Russian news agency RIA reported.

Residents and rescuers said dozens of towns and villages were in ruins from weeks of intensive aerial bombing.

The Russian-led campaign has also flattened dozens of hospitals and schools, international aid agencies said.

Trapped

Karen AbuZayd, a UN war crimes investigator on Syria, told reporters in Geneva that many of the destroyed or closed schools in opposition-held areas were now being used as shelters for people fleeing the violence.

The latest wave of displaced people came on top of close to 400,000 people who fled earlier for the safety of camps near the Turkish border, UN officials said.

Paulo Pinheiro, chairman of the UN Commission of Inquiry on Syria, told reporters on Thursday that many uprooted families now in makeshift camps were running low on food and water.

The latest offensive has brought the military campaign closer to heavily populated parts of Idlib province, where nearly three million people are trapped, according to the UN.

Rescuers and residents said Russian and Syrian jets pounded the devastated city of Maarat al-Numan on Thursday. It is one of the main urban centres in the province and lies on the main highway held by rebels.

The army and Iranian-backed militias are advancing towards the city, the capture of which would be a strategic government gain in the current campaign, which is intended also to regain control of major trade arteries important to Syria's war-torn economy.

On Wednesday, at least 21 civilians were killed in heavy aerial attacks, among them 19 people who died when bombs were dropped on a busy market place in the centre of Idlib city, the provincial capital.