Idlib, which borders Turkey, is home to nearly three million people, up to half of whom are rebels and civilians transferred en masse from other territory that has fallen to Syrian troops after intense assaults.

mobilising for a military offensive to retake Idlib, the last province still held by the country's beleaguered rebels.

His comments came amid fears the Syrian government, backed by long-time ally Russia, is

"I am once again prepared... personally and physically to get involved myself... to ensure such a temporary corridor would be feasible and guaranteed for the people so that they can then return to their own places once this is over," Staffan de Mistura told reporters in Geneva.

The UN's Syria peace envoy offered Thursday to travel to Idlib to help ensure civilians can leave through a humanitarian corridor amid fears of an imminent government offensive to retake the last major region controlled by rebels.

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

"There is no other Idlib," de Mistura said, stressing the need to ensure civilians can evacuate to nearby areas under government control, with guarantees their rights will be respected once they get there.

"It would be a tragic irony frankly if at almost the end of... a territorial war inside Syria, we would be witnessing the most horrific tragedy to the largest number of civilians," he said.

"It would be quite tragic at this stage, having seen how difficult the seven years (of Syria's war) have been."

More than 350,000 people have been killed and millions displaced since Syria's war started in 2011 with the brutal repression of anti-government protests.

Two years ago, de Mistura offered to go to eastern Aleppo and to personally escort Al Nusra fighters out of the besieged city.

"Al Nusra refused my offer to accompany them out, and they went to Idlib, and we lost two months at least and thousands of people died because of that," he said.

The UN envoy said there were an estimated 10,000 Al Qaeda and Al Nusra fighters in Idlib, along with their families.

While he stressed the legitimacy of battling such "UN-identified terrorists", he insisted efforts to defeat them did not justify putting the lives of some 2.9 million people in the area at risk.

"There is and can be no justification... to not avoid using heavy weapons in densely populated areas," he said.

On Wednesday, UN chief Antonio Guterres warned that full-scale military operations in Syria's

Idlib province could lead to a "humanitarian catastrophe" and cautioned against the use of chemical weapons.