The UN's new humanitarian chief warned on Monday that a large-scale military operation against the rebel-held Syrian province of Idlib could create the worst humanitarian catastrophe of this century.

A general view taken with a drone shows part of the rebel-held Idlib city, Syria

Geneva:

"There needs to be ways of dealing with this problem that don't turn the next few months in Idlib into the worst humanitarian catastrophe with the biggest loss of life in the 21st century," Mark Lowcock told reporters in Geneva.

His remarks came as Syrian troops, backed by Russia and Iran, massed around the northwestern province ahead of an expected onslaught against the largest rebel-held zone left in the country.

Since 2015, Idlib has been home to a complex array of anti-regime forces: secular rebels, Islamists, Syrian jihadists with ties to Al-Qaeda -- and their foreign counterparts.

It is home to some three million people -- around half of them displaced from other parts of the country, according to the United Nations.

Lowcock acknowledged that "there is a large number of fighters there, including terrorists from proscribed organisations."

But he stressed that "there are 100 civilians, most of them women and children, for every fighter in Idlib."

"We are extremely alarmed at the situation, because of the number of people and the vulnerability of the people," he said, warning that "civilians are severely at risk".

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

While appealing to the warring sides in Syria to avoid a catastrophe, Lowcock said the UN and other aid organisations were all doing "very detailed planning" to be able to respond quickly in the case of a major assault on the province.

"We very actively preparing for the possibility that civilians move in huge numbers in multiple directions," he said.

He said that the UN had plans to reach up to 800,000 people who might be displaced, and were bracing for around 100,000 people to move into government-held areas and some 700,000 to initially flee within Idlib.

The UN's World Food Programme, he said had already prepositioned food stocks for some 850,000 people for the first week or so of any large-scale military operation.

"It is a very major preoccupation for us," he said.