Forces led by Tahrir al-Sham, formerly the Nusra Front, involved in operations in northwest of country, says war monitor
Rebel groups launched a large attack on government-held areas in northwestern Syria on Tuesday, the Syrian Observatory for Human Rights and a media outlet run by the Lebanese group Hezbollah reported.
The attack north of Hama city marks the biggest offensive in that area since March, and underlines the complications facing Russian-led diplomatic efforts to preserve ceasefires in western Syria.
Several groups participated in the well-planned attack on government-held villages, the Observatory said, including Tahrir al-Sham - an alliance of groups led by the group formerly known as the Nusra Front.
The Hezbollah-run military news outlet said the army and its allies were repelling what it described as a large-scale attack in the area by the Nusra Front and factions affiliated to it.
The Observatory said warplanes were carrying out air strikes in the area and fierce clashes were under way. Fighters had launched heavy bombardments of government-held positions, it said.
The rebels taking part in the assault included the Turkistan Islamic Party, and a group fighting under the banner of the Free Syrian Army, the Observatory said.
Fighters advanced within a few kilometres of the government-held city of Hama earlier this year, before the Syrian army and its allies retook the territory in April.
The northern Hama area is adjacent to the rebel-held province of Idlib, which is largely controlled by Nusra.
Ceasefires in western Syria - for years the main theatre of the Syrian civil war - have helped the Syrian army and its allies advance against Islamic State in the east, where government forces are battling the group at Deir Ezzor.
Last week, Russia, Iran and Turkey agreed at talks in Kazakhstan to post observers on the edge of "a de-escalation zone" in Idlib region.
Nusra denounced ceasefire talks in Kazakhstan, and vowed to keep fighting.