Beirut: Rebels and Islamist forces killed more than two dozen pro-government fighters in a joint attack in a northwestern stronghold of Syrian President Bashar Al Assad’s regime, a monitor said Tuesday.

In retaliation, the government and its ally Russia pounded rebel-held areas in the coastal province of Latakia and neighbouring Idlib on Tuesday, killing four civilians, the Syrian Observatory for Human Rights said.

It was the bloodiest opposition assault on the area in three years, according to the Britain-based monitor, which relies on a network of sources inside the country.

The surprise offensive late on Monday targeted a pair of villages and observation points in Latakia, near the Turkish border.

“At least 27 regime forces and allied fighters, including eight officers, were killed in fierce clashes and shelling in the village of Al Ateira,” said Observatory head Rami Abdul Rahman.

“They took Al Ateira and kicked out regime forces from several observation points after killing or wounding them.”

Another 40 government loyalists were wounded and six rebels were killed, he said.

Al Ateira lies about two kilometres south of the Turkish border.

The Observatory said the opposition fighters included local rebels from Latakia as well as more hardline forces.

Latakia is the heartland of Al Assad’s regime on the Mediterranean coast, and is home to the Hmeimeem airbase where Russian troops and warplanes have been based since 2015.

Forces opposed to Al Assad have repeatedly attacked the province since the uprising broke out in March 2011. They hold a northeastern sliver of territory bordering the neighbouring province of Idlib.

The regime and Russia bombarded that strip of land and parts of western Idlib with air strikes and artillery fire on Tuesday, killing four civilians, the monitor said.

That area falls under a de-escalation deal agreed last year between Russia, fellow regime backer Iran, and rebel ally Turkey.