BEIRUT (Reuters) - Jihadists launched an offensive against government-held parts of northwestern Syria near Hama on Tuesday in their biggest attack there since March, triggering heavy air strikes on rebel territory, 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.

Islamist militants who hold sway in Idlib reject the diplomacy, including a tripartite deal struck last week by Moscow, Tehran and Ankara to deploy 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 that rebels were making advances in the northern Hama countryside, 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 insurgents taking part in the assault included Tahrir al-Sham, the jihadist Turkistan Islamic Party, and rebels fighting under the banner of the Free Syrian Army. They had captured four villages, it said.

The ex-Nusra Front, which cut ties with al Qaeda and rebranded last year, spearheads the Tahrir al-Sham alliance of Islamist groups.

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 government-held Hama city earlier this year, before the Syrian army and its allies retook the territory in April.

Ceasefire deals in western Syria - for years the main theater of the war - have helped the Syrian army and its allies advance against Islamic State in the east, where government forces are battling IS in Deir al-Zor.

A U.S.-backed militia force, the Syrian Democratic Forces, is waging a separate offensive against Islamic State in Deir al-Zor province, 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 river 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 captured a village and parts of the nearby town of Khasham on the eastern bank.

Russian Defence Ministry spokesman Igor Konashenkov said Syrian soldiers, with Russian air power, “continued to expand the captured area” in recent days.

“Despite the persistent resistance of ISIS (Islamic State) fighters, Syrian troops managed to free more than 60 square kilometers of terrorists on the left bank of the Euphrates,” he said.