Syrian regime forces faced fierce resistance from jihadists Thursday as they fought to capture a key airbase in Idlib, the last province beyond the government's control, a war monitor said.

Russia-backed government forces battled their way into the Abu Duhur military airport on Wednesday night as they pressed a weeks-long offensive in the northwestern province.Idlib province is almost entirely held by Hayat Tahrir al-Sham, a jihadist outfit dominated by Al-Qaeda's former Syrian affiliate.

Government fighters have seized a string of villages in the southeast of the province since launching their offensive on December 25.

After entering the Abu Duhur airbase, regime forces faced "fierce resistance", the Syrian Observatory for Human Rights said.

And on Thursday, Hayat Tahrir al-Sham and its Islamist allies led a counter-attack, retaking several villages from the regime, said the Britain-based war monitor.

Later, they fired rockets at regime positions from farmland in the Al-Tamana area of southern Idlib, an AFP correspondent said.

The goal of the jihadists was to cut off regime forces at the airport from their rear bases and "relieve the pressure" on them at the airbase, said the Observatory.

Thursday's battle, which was punctuated by air strikes from Syria's regime and its ally Russia, claimed the lives of 26 jihadists and rebels and 15 pro-government fighters.

On Wednesday, 35 soldiers were killed at the airport, said the Observatory, which relies on a network of sources on the ground in the war-torn country.

The regime lost control of the Abu Duhur base in 2015 after a two-year siege by jihadists, with only the Shiite-populated villages of Fuaa and Kafraya remaining under its control in the whole province.

- Fear among displaced -

Idlib's few rebel groups and Hayat Tahrir al-Sham have set aside their differences to fight President Bashar al-Assad's forces.

The regime hopes to seize control of southeast parts of Idlib province to secure a main road between the capital Damascus and the northern city of Aleppo.

The fighting in Idlib has forced nearly 100,000 people to flee since early December, the United Nations says.

The UN's humanitarian coordination office OCHA said on Wednesday that the situation in Idlib was "extremely chaotic", with newly displaced people reportedly scattered across open areas.

"Due to the large numbers of people moving, many are left with no shelter, which could expose them to various risks, especially as the winter temperatures continue to drop," it said in a statement.

The International Rescue Committee said it had received hundreds of people newly displaced from the latest fighting in Idlib, many of them housed in makeshift tents.

One, a mother of twins, said she had initially left one of her children behind in the panic to escape the air strikes.

"We couldn't think properly. The fear affected our brains," said the mother, quoted by the IRC.

Ninety-six civilians, including 27 children, have been killed since December 25 in Syrian or Russian air strikes on Idlib, according to the Observatory.

The violence comes as UN humanitarian chief Mark Lowcock wraps up a visit to Syria during which he met with authorities.

Lowcock said he was "deeply worried about civilians affected by the upsurge in violence in Idlib".

Idlib, which borders Turkey, is one of four "de-escalation" zones in Syria covered by a deal meant to reduce violence that was struck last year by Turkey, Russia and government-backer Iran.

The regime's Russian-backed offensive has angered Ankara, which on Wednesday called on Moscow and Tehran to "fulfil their responsibility" and halt the operation.

Turkey on Tuesday summoned the Russian and Iranian ambassadors to Ankara in order to convey its "uneasiness" about their actions in Idlib.

"Regime forces are striking moderate opposition with the pretext of fighting against Al-Nusra (Front)," Foreign Minister Mevlut Cavusoglu said, referring to the former Al-Qaeda affiliate.

In a statement from the Syrian presidency Thursday, Assad said the "major victories of the army in cooperation with Russia and its other allies... have strengthened efforts for a peaceful solution" to the conflict.

More than 340,000 people are estimated to have been killed in the Syrian war, which began in 2011 as the regime brutally crushed anti-government protests. Millions have been displaced.