UN Syria envoy Staffan de Mistura said a potential "perfect storm" is looming over Syria's Idlib province, with possible military implications beyond the region.

Speaking to reporters in Geneva on Thursday, De Mistura offered 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.

"You can understand that when there is a perfect storm coming up in front of our eyes potentially, we need to address first things first," De Mistura said.

"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," he added.

The UN also called on Russia, Iran and Turkey to forestall the battle in Idlib, which would affect millions of civilians and could see both sides using chemical weapons.

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 territories that has fallen to Syrian troops after intense battles.

A major military operation in Idlib would pose a particularly threatening humanitarian situation 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."

'Hotbed of terrorists'

Meanwhile, the Kremlin on Thursday called the Syrian rebel-held province of Idlib a "hotbed of terrorists" and said not tackling the problem was not a good option.

Kremlin spokesman Dmitry Peskov made the comments to reporters on a conference call while answering a question about planned Russian naval drills in the Mediterranean, an exercise he said was justified due to the difficult situation in Syria.

Russia said on Wednesday that fighters there "must be liquidated" and Foreign Minister Sergey Lavrov described them as "a festering abscess".

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

A major offensive in the Idlib area, where displaced people already make up half the population, risks forcing another 700,000 Syrians from their homes, the United Nations said.

It also risks raising tensions with Turkey, whose army established a ring of observation posts around the rebel territory last year under a "de-escalation" deal with Russia and Iran.

Turkey, which has a small military presence in Idlib, has warned against such an attack.