Ten of thousands of people have fled to the Turkish border in the last few days as the Syrian army pushed a major advance further into the opposition's last major stronghold, residents, rights groups and opposition sources said on Wednesday.

They left Maarat al-Numan, a main city in Idlib province that has been a sanctuary for families fleeing former rebel areas, as a Russian-led campaign has come close to capturing the strategic town of Khan Sheikhoun further to the south.

"The flow of cars and vehicles leaving is not stopping," said Abdullah Younis from the city. Rescuers there said around 60,000 people had fled in the last four days alone.

>> Read more: As U.S. and Turkey argue over Idlib, Syria's wild south reawakens | Analysis

On Tuesday, Russian and Syrian jets intensified their bombing of scattered villages and towns around Maarat al-Numan, with the al-Rahma hospital in the area struck, residents said.

Open gallery view Map of the areas of control in Syria according to Carter Center Credit: Reuters

"There were 15 raids on Jarjanaz in less than five minutes," Abdul Rahman al Halabi told Reuters from the area.

Rebels concede most of their fighters have fled Khan Sheikhoun but are providing fierce resistance to the Syrian army, which has secured a foothold in the town.

State media, broadcasting from the outskirts of Khan Sheikhoun on Tuesday, said government forces were battling militants but had extended their advance and seized a highway running through the town.

Capturing Khan Sheikhoun would be an important gain for Moscow and its ally into the northwestern region, where Moscow has helped President Bashar al Assad turn the tide against insurgents in the eight year conflict since stepping up its intervention in 2015.

Russia has thrown its weight behind the campaign, which began in earnest at the end of April, conducting thousands of raids and strikes on rebel-held northern Hama and southern Idlib in what Western military experts and opposition figures say is a "scorched earth strategy".

Foreign Minister Sergei Lavrov acknowledged on Tuesday that Russia had military personnel on the ground in Idlib province, the Interfax news agency quoted him as saying.

The Russian military has previously downplayed its direct role in the campaign, where it used mercenaries and special forces as well as directing battles, according to Western intelligence sources.

After months of stalemate Russia has increased the intensity of raids in the last 10 days, dramatically transforming the situation on the ground. Hundreds of civilians have been killed and at least 400,000 people displaced, according to medics and NGOs and the United Nations.

Moscow and Damascus, who deny indiscriminate bombing of civilians areas, say they are fighting jihadist militants.