BEIRUT (Reuters) – Jihadists launched an offensive against government-held areas of northwestern Syria near Hama on Tuesday in their biggest attack in the area since March, triggering heavy air strikes on rebel-held areas, the Syrian Observatory for Human Rights said.

It said air strikes hit three hospitals, a medical center and premises used by a rescue service in rebel-held Idlib. A Syrian military source denied the report, saying only insurgent convoys and positions had been hit.

The insurgent attack north of Hama revived hostilities in the northwestern region near the Turkish border that has been relatively calm in recent months as Russian-led diplomacy seeks to shore up ceasefires in western Syria.

The Islamist militants who hold sway in Idlib reject the diplomacy, including a tripartite deal struck last week by Moscow, Tehran and Ankara for a deployment of an observer force on the edge of an Idlib “de-escalation zone”.

A Syrian army source cited by state media said the attack launched on several fronts was being repelled, and the insurgents had suffered losses.

“The clashes are continuing and the air force and artillery are targeting the headquarters and movements of the terrorist convoys in the area,” said the source.

An insurgent source told Reuters the rebels were making advances in the northern Hama countryside, situated in an area where President Bashar al-Assad and his allies have been steadily rolling back rebel gains over the last two years.

The Observatory said they had captured two villages. The insurgents taking part in the assault included the jihadist Turkistan Islamic Party and rebels fighting under the banner of the Free Syrian Army, it said.

A media outlet run by the Damascus-allied Lebanese group Hezbollah said Syrian army air strikes were targeting insurgents in the northern Hama and southern Idlib area.

Insurgents advanced to within a few km (miles) of the government-held city of Hama earlier this year, before the Syrian army and its allies retook the territory in April.

Ceasefires in western Syria – for years the main theater of the country’s civil war – have helped the Syrian army and its allies advance against Islamic State in the east, where government forces are battling IS at Deir al-Zor.

A U.S.-backed militia force, the Syrian Democratic Forces, is waging a separate campaign against Islamic State in the Deir al-Zor area, focusing on areas on the eastern bank of the Euphrates River.

The rival forces have generally stayed out of each other’s way, with the Euphrates often acting as a dividing line.

Syrian government forces and their allies have however crossed into the SDF’s area of operations on the eastern bank of in recent days. The Hezbollah-run media unit said on Tuesday that government forces and their allies had captured a village and parts of the nearby town of Khasham on the eastern bank.