The US-backed Syrian Free Army has taken control of the city of Palmyra in eastern Homs province, following clashes with Syrian regime forces loyal to President Bashar al-Assad.
The group took control of the ancient city after sweeping through previously regime-held areas in Homs's eastern countryside.
The Syrian Free Army also now controls the town of Al-Sukhnah, situated between Deir ez-Zor and Homs, and Al-Qaryatayn village.
Earlier in the day, it was reported that regime forces had abandoned their positions at the key T-4 airbase near Palmyra.
The capture of Palmyra, which was previously held by the Islamic State group and later re-taken by the Syrian regime, comes amid a lightning offensive by Syrian opposition groups against the Damascus-based regime which began on 27 November.
The Hayat Tahrir al-Sham rebel group captured the key northern city of Aleppo in an offensive between 27-30 November, leading to the collapse of regime control in several other areas of the coutnry.
The Turkish-backed Syrian National Army (SNA) on 1 December launched 'Operation Dawn of Freedom' against Syrian regime forces and the Kurdish-led Syrian Democratic Forces in northern Aleppo governorate.
Rebel forces captured the city of Hama, dubbed the 'capital of the revolution' on Thursday, and have now made advances on the city of Homs and the capital Damascus.
Syria's interior minister told state TV that security forces had imposed an impenetrable cordon around Damascus on Saturday, as fighters opposing the government said they were nearing the capital.
"There is a very strong security and military cordon on the far edges of Damascus and its countryside, and no one... can penetrate this defensive line that we, the armed forces, are building," Interior Minister Mohammed al-Rahmoun told state TV from Damascus.
The reigniting of Syria's war comes more than a decade after protests broke out in the country demanding that Assad step down.
The Syrian leader brutally suppressed the uprising, leading to a protracted civil war in the country.
After initial gains by rebel groups, the Syrian regime clawed back areas of the country with backing from Russia, Iran and the Lebanese Hezbollah group.