Forces loyal to President Bashar al-Assad fought rebels in a small town 100 km (60 miles) north of Damascus on Thursday after video footage showed the opposition captured a huge weapons cache, Reuters reported.
The looting of hundreds of weapons in the town of Mahin will aid rebels who are based there, halfway between the capital and Homs, two cities where the opposition has tried to take territory during the civil war.
State television said on Wednesday evening that the army was engaging with "terrorists" in Mahin, its usual description for the rebels. An anti-Assad monitoring group said that four rebel fighters had been killed on Thursday.
The group, the Syrian Observatory for Human Rights, said several brigades, including hard-line Islamists linked to al Qaeda, were taking part in the fighting.
Opposition video footage from Tuesday showed rebels fighting in the desert and shots of militants standing in front of hundreds of green crates, some open with munitions showing.
An opposition activist in the video said the depot included missiles, artillery shells and mortars.
"These mortars are very long," he said, looking into an open green box. "Only the Assad brigades have these."
Another rebel in a video posted online on Thursday stood in front of a tank, saying it was one of three that opposition forces had taken from Assad's forces in Mahin.
Reuters could not confirm the authenticity of the videos nor when they were filmed.
Eliot Higgins, a UK-based researcher who trawls daily through online videos of Syria's civil war and verifies weapons in them, said he had not seen a "big ammo cache like this for several months."
He said that in addition to mortars, artillery and tank ammunition, the size and length of some of the boxes in one video posted online by rebels indicated there were rockets.
"This will help (the rebels') war efforts. In that one room there is a very significant amount but, depending on the intensity of the fighting, they could burn through these in a couple of weeks."
After 2-1/2 years of war, which started when Assad's forces fired on pro-democracy protests, the fighting has settled into a broad stalemate in which more than 100 die every day, with few significant advances by either side.
More than 100,000 have died since the start of the conflict, the United Nations says.