They said government forces captured Qalaat al-Madiq and two nearby villages, Tal Hawash and al-Karkat, on Thursday.
The so-called Syrian Observatory for Human Rights said militants had withdrawn after being nearly encircled by the army, Presstv Reported.
The recapture of Qalaat al-Madiq on the southwestern edge of the Idlib region comes after government forces retook the nearby town of Kafr Nabouda on Wednesday.
Syria's army launched ground operations this week against the militant-held zone consisting of Idlib and parts of adjacent provinces.
Turkey and Russia brokered a deal in September 2018 to create a demilitarized zone in the region that would be evacuated of all heavy weapons and militants.
However, terrorist outfits especially militants from the Jabhat Fateh al-Sham terror group formerly known as al-Nusra Front constantly breach the agreement by attacking civilians and carrying out provocations against the Syrian army.
Syrian President Bashar al-Assad has recently called for the removal of any "obstacles" blocking the full implementation of the deal. He also stressed his government’s determination to eliminate terrorist groups holed up in Idlib.
Russian Foreign Minister Sergei Lavrov said last month that the remnants of Daesh and al-Qaeda affiliate Jabhat al-Nusra were reluctant to abide by the ceasefire agreement in Idlib.
On Tuesday, Russia said its surface-to-air missile systems had managed to foil an attack by Takfiri militants against the Hmeimim airbase in Syria's coastal province of Latakia.
Russia has been helping Syrian forces in the ongoing battles across the Arab country.
Syria has been gripped by foreign-backed militancy since March 2011. The Syrian government says the Israeli regime and its Western and regional allies are aiding Takfiri terrorist groups that are wreaking havoc in the country.