Fighting erupted in Damascus on Sunday near a complex linked to Syria's chemical weapons programme, on the third day of an offensive by President Bashar al-Assad's forces aimed at driving rebels from main sectors of the capital, activists said.
The fighting occurred near the Scientific Studies and Research Centre on the foothills of Qasioun Mountain in the northern Barzeh district, opposition sources said from Damascus.
Barzeh is one of several working class neighborhoods that have turned into footholds for opposition brigades, who have infiltrated Damascus from swathes of farmland dotted with built-up areas on the outskirts of Damascus known as al-Ghouta.
The rebels lack the firepower to breach the heavily fortified Research Centre complex and the compound is being used to shell Barzeh, the sources said.
The U.S. administration said last week that Assad's forces had probably used chemical arms in the conflict and congressional pressure has mounted on the White House to do more to help the rebels.
Republican senators on Sunday pressed President Barack Obama to intervene, saying America could attack Syrian air bases with missiles but should not send in ground troops.
Neutralizing Assad's air advantage over the rebels "could turn the tide of battle pretty quickly," Republican Senator Lindsey Graham told a CBS news programme.
In Barzeh at least nine people were killed and 70 were wounded in the last three days, mostly from army shelling. The district is home to a military hospital, hit by rocket-propelled grenades and mortar rounds on Sunday, and an electronic eavesdropping facility, as well as a military police compound and another army unit, the sources said.
Syrian warplanes bombed on Sunday the adjacent district of Qaboun, through which Barzeh is being supplied from the Ghouta. There were no immediate reports of casualties, according to activists in the neighborhood.
The Syrian official state news agency said "units of the heroic Syrian army have inflicted heavy losses on terrorists" in Barzeh, eastern Damascus and Ghouta.
Speaking form Barzeh, opposition activist Abu Ammar said the research center was the only military facility in Barzeh that the rebels have not managed to hit. He added that a chemical weapons storage facility is located near the center "It is very heavily fortified and there are heavy caliber anti-aircraft guns deployed in the complex and in large tracts of land that are part of it," he said.
He said opposition fighters in Barzeh repulsed an attack on their strongholds in the district from the adjacent Ush al-Warwar area, part of several hilltop enclaves inhabited by Assad's minority Alawite sect.
"Barzeh has been besieged for the last fifty days; with a narrow supply line to Ghouta through Qaboun," Abu Ammar said.
"Fighting has intensified in the last three days and the regime sent down his militia today from Ush al-Warwar but the fighters forced them to turn back," he added.
Activists reported fighting in the nearby district of Jobar to the south, where an air strike near a mosque set off a huge plume of white smoke, according to video footage taken by the opposition, as fighting continued across the Ghouta.
The army seized the town of Otaiba, near the Damascus International Airport, in Ghouta last week, cutting a weapons supply route into the eastern fringes of Damascus that rebels had used for eight months.
Syria's uprising is the bloodiest and longest of Arab revolts that erupted more than two years ago.
It began with peaceful protests against Assad that were met with force, sparking armed opposition and eventually civil war pitting Assad's minority Alawite sect against the Sunni Muslim majority.
The army appears to have made gains in the north and center of the country in recent weeks.