The Syrian army bombarded rebel strongholds in southern Damascus on Monday with artillery and from the air, hours after opposition fighters attacked a militia loyal to President Bashar Assad, opposition activists said.
At least eight people were killed and dozens wounded in the bombardment, after 20 people were killed by army shelling overnight, they said.
Warplanes fired rockets and tanks and artillery pounded the neighborhoods of Sbeineh, Yalda, Bibla, al-Tadamun and Hajar a-Aswad on Monday, the activists said.
The working-class Sunni Muslim neighborhoods have been at the forefront of the 19-month-old revolt against Assad, whose Alawite faith derives from Shi'ite Islam.
Rebel Free Syrian Army fighters attacked a pro-Assad militia in the southern neighborhood of Nisreen overnight, an area mainly populated by members of Assad's minority Alawite sect.
Rebels also hit positions belonging to the Popular Front For the Liberation of Palestine-General Command (PFLP-GC), an Assad proxy, in the nearby Palestinian refugee camp of Yarmouk.
At least seven PFLP-GC members were killed in the fighting, and ambulances were seen taking dozens of casualties from Nisreen to a nearby hospital, activists in the area said.
Meanwhile, Russian Foreign Minister Sergei Lavrov told Egypt's state al-Ahram daily in an interview published on Monday that Moscow is still supplying arms to Syria under Soviet-era commitments.
Russia sold the Syrian government $1 billion worth of weapons last year and has made clear it would oppose an arms embargo in the United Nations Security Council, contending that rebels would get weapons illegally anyway.
Lavrov claimed, however, that the arms being sent to Damascus were part of old Soviet contracts and did not violate international law.
"As for the Russian-Syrian technical military cooperation, it aims to support Syria's defense capabilities in the face of external political threat, and not to back Bashar Assad," Lavrov was quoted as saying.
He also accused foreign powers of arming the opposition to topple the government in breach of international law, adding that such weapons could fall into the hands of al-Qaida fighters.
Western powers back the rebels but say they have stopped short of sending arms. Qatar, which has been an outspoken critic of Assad and called for a no-fly zone, has also denied providing arms but says it does give logistical and humanitarian support.
"It was the Soviet Union that supplied Syria with main weapons but at present we are in the process of finalizing the implementation of our commitments which are linked primarily to the supply of some air defense systems," Lavrov told al-Ahram.
"These military exports are of a defensive nature and do not conflict with international treaties," he said.
A Russian official said in July that Moscow would not deliver fighter planes or other new weapons to Syria while the conflict there remained unresolved.
Russia and China, both permanent Security Council members, have vetoed three Western-backed UN draft resolutions condemning Assad's government for its handling of the uprising that began with peaceful demonstrations in March 2011.
The protests turned into an armed revolt after Assad used force to crush opposition. About 32,000 people have been killed.