Fighters from the Ansar Al Tawhid group and the Turkestan Islamic Party attacked troops in the south of Idlib province, the Syrian Observatory for Human Rights said, killing at least 11 and wounding 20 others.
Militants detonated explosives in specially-built tunnels beneath army positions and launched a simultaneous attack from other tunnels, the Observatory's director Rami Abdel Rahman.
Syria is witnessing some of the worst violence in years as fighting flares in Idlib and the southern region of Deraa.
Damascus and Russia have stepped up shelling, while rebel and militant groups have also increased attacks on Syrian army troops and fighters from Iran-backed groups close to President Bashar Al Assad.
ISIS has also staged attacks, while Israel has bombed Iran-linked facilities near the capital, killing dozens of fighters.
Civilians have also been targeted, with two killed in Russian air strikes at an abandoned water pumping station on Wednesday.
A woman and two children were also injured.
Moscow has increased bombing in the rebel-held north-west, swathes of which are controlled by the rebel group Hayat Tahrir Al Sham, headed by former Al Qaeda-affiliated fighters.
Both groups involved in Saturday’s attack are affiliated with HTS, which controls parts of Latakia, Aleppo and Hama provinces.
The war monitor said the death toll was expected to rise as the “intense clashes are still continuing”.
Seven HTS fighters were killed on Friday in bombardments by government forces and at least 13 others in Russian air strikes on Monday in northern Syria, said the Observatory, which relies on a network of sources on the ground in Syria for its reports.
The Syrian war has killed more than 500,000 people, forcing around half of the country's prewar population to flee their homes.