The United Nations called on Russia, Iran and Turkey on Thursday to forestall a battle in Syria's Idlib province which would affect millions of civilians and could see both sides using chlorine as a chemical weapon.

UN Syria envoy Staffan de Mistura told reporters that there was a high concentration of foreign fighters in Idlib, including an estimated 10,000 terrorists, but it would be better to set up humanitarian corridors to evacuate civilians than rush into a battle which could turn prove to be a "perfect storm".

The Kremlin earlier on Thursday called the Syrian rebel-held province 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 the difficult situation in Syria justified.

Russia plans to hold a major naval exercise in the Mediterranean Sea from September 1 to September 8, the TASS news agency reported.

TASS cited the defence ministry as saying that 25 warships and submarines and 30 planes, including fighter jets and strategic bombers, would take part, and that the drills would involve anti-aircraft, anti-submarine and anti-mining exercises.

A source said on Wednesday that Russia's ally, Syrian President Bashar al-Assad, was preparing a phased offensive in Idlib.

Russian Foreign Minister Sergei Lavrov said on Wednesday that militants in Idlib had to be liquidated, describing them as "a festering abscess".

"In the interest of ensuring the security of shipping and aircraft flights in line with international law, the areas of the exercise will be declared dangerous for shipping and flights," TASS quoted the defence ministry as saying.

Anatoly Antonov, Russia's Ambassador to the United States, said earlier on Thursday that he had told US officials that Moscow was concerned by signs that the United States was preparing new strikes on Syria.