Syria's army on Wednesday entered Al-Hosn in the central province of Homs and was fighting for control of the famed Krak des Chevaliers castle, a security official told AFP.
Krak des Chevaliers, built between 1142 and 1271 and considered one of the best preserved Crusader castles in existence, has already sustained damage from previous fighting between rebels and regime forces during Syria's three-year conflict.
"The army entered Al-Hosn and took two districts of the village. It is bombarding areas around the Krak des Chevaliers to take control of the castle," the official said, speaking on condition of anonymity.
State news agency SANA meanwhile said the army had carried out an ambush of opposition forces west of the nearby town of Zara, which it captured on March 8.
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The army was also engaged in continuing clashes in the Qalamoun region, along the border with Lebanon, where it captured the rebel bastion of Yabrud on Sunday.
The Syrian Observatory for Human Rights, a Britain-based NGO, reported fierce fighting between regime and rebel forces at a checkpoint near the town of Rankus, south of Yabrud.
It said regime troops backed by Lebanon's Hezbollah and pro-regime militia were also fighting rebel and jihadist forces around the villages of Bkhaa and Jobeh in Qalamoun region.
Elsewhere, the Observatory said rebels had taken control of the central Gharaz prison in southern Daraa province, freeing an unspecified number of prisoners.
And in Eastern Ghouta, a suburb of the capital Damascus, the Observatory said two children had died because of food and medical shortages and dire living conditions caused by a regime siege of more than a year.
Government troops have laid siege to several areas in the country under rebel control, causing shortages that have resulted in dozens of deaths.