By the end of last week, they had advanced into Idlib, close to an insurgent-held military airport.
The al-Qaida-linked Levant Liberation Committee (LLC) refused to join the two rebel operation parts, and chose to fight the Syrian army alone, according to the report. The Syrian army, supported by Iran-backed militias and Russian air power, began an offensive in late October in Hama province.
The regime's goal is to retake the southeast of Idlib province in order to secure a route between Damascus and second city Aleppo, both under its control, according to the Observatory.
Rebels launched a counter attack on Thursday.
Since losing the airbase in 2015 following a two-year jihadist siege, the Syrian government has only had control of two villages in Idlib province: Fua and Kafraya.
Turkish President Tayyip Erdogan told Russian counterpart Vladimir Putin on Thursday he should stop Syrian attacks on its opposition in northern Syria if he wants peace negotiations to succeed, Turkish presidential sources said.
A military media unit run by the Assad regime's Lebanese ally Hezbollah, and a Britain-based war monitor, the Syrian Observatory for Human Rights, said on Friday the army had recaptured several villages taken in the rebel counterattack.
"Residential areas are being hit day and night by strikes from the ground and from the air, forcing civilians to hide in basements", he said in a statement.
Idlib province is nearly entirely controlled by Hayat Tahrir al-Sham, a jihadist outfit dominated by Al-Qaeda's former Syrian affiliate.
Almost 100,000 people have been displaced since early December by fighting in northwestern Syria, where the regime is waging an offensive against jihadis, the United Nations said on Wednesday.