Syrian troops attacked rebel positions outside Khan al-Assal on Wednesday as they seek to recapture the northern town at the centre of rival chemical weapons accusations, a watchdog said.
Fierce fighting erupted on the outskirts of the town, which the rebels seized on July 22 week inflicting heavy losses on the army, the Syrian Observatory for Human Rights said.
The army lost 150 soldiers over two days, 50 of them summarily executed after their capture, in an act condemned by the mainstream opposition leadership.
Khan al-Assal was the scene of what both the government and the opposition say was a chemical weapons attack that killed 30 people on March 19.
The Damascus government has pressed for Khan al-Assal to be the focus of a planned UN inquiry into a total of 13 allegations of chemical weapons use during Syria's 28-month civil war.
The rebels' capture of the town last week came as two UN envoys visited Damascus to thrash out the ground rules for UN weapons inspectors. They came away with an agreement but UN chief Ban Ki-moon has said he is still reviewing the details.
Signup to our newsletter and follow us on Facebook and Twitter!
The oil refinery of the central city of Homs came under fire on Wednesday for the second time in a week, the Observatory said. Several of the refinery's staff were wounded by "terrorist rocket fire," the state SANA news agency said.
As UN efforts to convene a Russian- and US-backed peace conference have faltered, government forces have launched counter-offensives in the centre and north, as well as around the capital.
The opposition has vowed to step up efforts to capture the remaining government-held districts of second city Aleppo.
Six workers of a "scientific research centre" -- or military research facility linked to the defence ministry -- in Barzeh, a district of north Damascus, were killed and 19 wounded when rebels fired a mortar round at their bus, SANA said.
The Observatory gave a casualty toll of four dead and 20 wounded.
Elsewhere, the Observatory reported, two local dignitaries who have often served as mediators between rebels and loyalist forces were gunned down on Tuesday night in the northwestern city of Zabadani.