"Jihadist factions led by Hurras al-Deen launched an assault on two villages in Sahl al-Ghab," a region of the central province of Hama, the Syrian Observatory for Human Rights said.

They briefly seized the villages of Al-Fatatra and Al-Manara, Observatory chief Rami Abdel Rahman said.

But regime forces took them back just hours later amid Russian air strikes and intense shelling, the Britain-based monitor said.

Abdel Rahman said the clashes left 19 government soldiers and 22 jihadists dead.

The country's official SANA news agency also reported the attack on "two Syrian army positions," saying it involved "explosive-rigged vehicles and suicide bombers".

Citing a military source, it said the situation was now under control after the jihadists were forced to pull out under regime fire.  

The offensive was launched from within the Idlib region, which is controlled by jihadists and rebels and is the last bastion of resistance to Damascus after nine years of devastating civil war.

A truce reached on March 6 has largely stemmed fighting in the region, which President Bashar al-Assad has vowed to fully retake.

Hurras al-Deen is a relatively small but powerful armed group led by Al-Qaeda loyalists.

It is a rival of Hayat Tahrir al-Sham, an alliance of Islamist factions dominated by former members of Al-Qaeda's ex-Syria affiliate Jabhat al-Nusra -- although the two groups cooperate at times.

The Idlib truce brokered by regime ally Russia and rebel backer Turkey has largely kept Syrian and Russian warplanes out of the region's skies.

The calm came as the novel coronavirus pandemic spread across the Middle East.

Aid groups had warned that an outbreak of the COVID-19 illness in the Idlib region could cause a humanitarian disaster of previously unseen proportions.