Syrian government forces clashed with rebels in Hama province on Friday in some of the heaviest fighting in the northwest in a year, the Syrian Observatory for Human Rights said.
The army and allied forces attacked insurgents near Halfaya village overnight, seizing some positions, the UK-based war monitoring group said.
State news agency SANA said soldiers killed militants in an ambush in Hama "in response to their attacks on a military position with heavy machine guns".
At least 22 members of the Jaish al-Izza faction died in ambushes, and dozens more were wounded, the highest casualty toll of fighters in the northwest in many months, it said.
The Observatory said it was unclear if pro-government forces were killed.
Intermittent exchanges of fire have broken out in northwest Syria since a deal in September between Russia, a key Damascus ally, and Turkey, which has backed the opposition.
The agreement to set up a demilitarized zone staved off an army offensive against the Idlib region under insurgent control, including nearby parts of Hama and Aleppo provinces.