Beirut (Lebanon) (AFP) - Fierce clashes between Russia-backed government forces and jihadists have killed more than 100 combatants in northwestern Syria in the past 24 hours, a war monitor said on Friday.

The clashes on the edge of the jihadist-controlled Idlib region have killed at least 53 government loyalists along with 48 jihadists and Islamist fighters since Thursday, the Britain-based Syrian Observatory for Human Rights said.

The region of some three million people -- almost half of them displaced from other parts of the country -- is dominated by the Hayat Tahrir al-Sham alliance led by Al-Qaeda's former Syrian affiliate.

The alliance administers a region that spans most of Idlib province as well as adjacent slivers of the neighbouring Latakia, Aleppo, and Hama provinces.

The region has seen a spike in bombardment since late April, with regime forces seizing several towns on the region's southwestern flank.

Late Thursday, the jihadists and allied rebels launched a counterattack against regime forces in the northwest of Hama province.

They have since seized the villages of Tal Maleh and Jibeen amid heavy clashes and airstrikes by regime and Russian forces, the Observatory said.

The area saw "the fiercest fighting since the start of the escalation in late April," Observatory chief Rami Abdel Rahman said.

The fighting is raging near Christian and Alawite areas under regime control, he said.

State news agency SANA said army units in northern Hama had "responded with force to attacking terrorist groups... and took back a number of positions".

In the Idlib province, two children were killed overnight -- one in regime rocket fire in the village of Frike and another in an airstrike in the town of Khan Sheikhun, the Observatory said.

The jihadists who took part in the counterattack include some from the Al-Qaeda-linked Hurras al-Deen and Turkistan Islamic Party.

Idlib is supposed to be protected from a massive regime offensive by a September buffer zone deal signed between regime ally Russia and rebel backer Turkey.

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But it was never fully implemented, as jihadists refused to withdraw from the planned demilitarised zone.

The government and Russia have upped their bombardment of the region since late April, killing more than 300 civilians, according to the Observatory.

The United Nations say more than 270,000 people have fled their homes.

A total of 24 health facilities and 35 schools have been hit in the latest escalation, according to the UN's humanitarian office.

Analysts predict that Syrian President Bashar al-Assad and his allies will continue to chip away at the area, but not unleash a major assault that would create chaos on Turkey's doorstep.

Eight years into Syria's civil war, two regions largely remain beyond regime control: a large northeastern swathe held by the country's Kurds, and Idlib.

The war, which started in 2011 with the brutal repression of anti-government demonstrations, has killed more than 370,000 people and displaced millions.