The UN urged Syria Friday not to "revictimise" the children of fighters in Idlib amid a feared government offensive on the rebel-held province, insisting they were not responsible for their parents' actions.

The United Nations children's agency, UNICEF, voiced alarm at the likely impact of a major military operation in the province, which is home to some three million people -- a third of them children.

"The greatest concern is that among the very strong military rhetoric at the moment, we seem to forget that there are more than one million children living in that area," UNICEF's director for emergency operations Manuel Fontaine told AFP in an interview.

Fontaine was just back from a trip to Syria, where he travelled to Damascus, Homs and Aleppo but did not gain access to Idlib.

He voiced particular concern for the children of rebel fighters in that province, worrying that they might suffer the consequences of their parents' choices.

"I think there is a real risk to revictimise these children and again associate them to what their fathers might have done," said Fontaine.

"It is extremely important that common sense prevails... and that frankly, when you are a four-year-old, a five-year-old, a 10-year-old or 11-year-old, and your parents have made a particular decision, that is not your responsibility," he said.

"The protection of these children is extremely important to us, and we will certainly be watching over that very carefully."

- 'Particularly vulnerable' -

Fontaine's comments came amid fears of an imminent government offensive on Syria's only remaining rebel-held province.

Government forces looked poised to launch what could be one of the last major battles of the civil war that has torn Syria apart since 2011.

After retaking a succession of rebel bastions around the country this year, the government of President Bashar al-Assad has set its sights on Idlib.

The province, which borders Turkey, is home to nearly three million people, up to half of whom are rebels and civilians transferred en masse after pro-government forces retook formerly rebel-held areas.

"We know that in Idlib, there are children who have been displaced five, six, seven times already," Fontaine said, pointing out that many had already lived through massive military attacks in places like Aleppo, Homs and Eastern Ghouta.

"They are particularly vulnerable," he said.

A major military operation in the province is expected to pose a particular humanitarian nightmare because there is no nearby opposition territory left in Syria where people could be evacuated to.

UN envoy Staffan de Mistura called Thursday for the creation of a "humanitarian corridor" to allow civilians to evacuate to nearby areas under government control.

Fontaine stressed that anyone opting to leave "would have to be protected and provided immediate humanitarian assistance.

"We need to be given access to them very rapidly so that we can help."

He also stressed that "any movement needs to be voluntary," and that "those who decide to stay still need to be protected. They are civilians, and they shouldn't be put in harm's way."

He said UNICEF was laying contingency plans "to provide immediate assistance to up to 400,000 displaced people."

The UN humanitarian organisation has meanwhile estimated that as many as 800,000 people might flee in one of the Syrian war's largest displacements yet.