Artillery and helicopters hammered the Sunni Muslim town of Daraya for 24 hours, killing 15 people and wounding 150, before soldiers moved in and raided houses, opposition sources said.
About 100 people, including 59 civilians, were killed in violence across the country, according to the opposition Syrian Observatory for Human Rights. Some 200 were killed on Wednesday.
There was little resistance as Assad's forces pushed toward the centre of Daraya, on the southwest edge of Damascus. Armed rebels had apparently already left, activists in Damascus said.
"They are using mortar bombs to clear each sector. Then they enter it, while moving towards the centre," said Abu Zeid, an activist speaking by phone from an area near Daraya.
Assad's military had driven insurgents from most of the areas they seized in and around the capital after a bomb killed four top security officials on July 18. But rebels have crept back, regrouping without taking on the army in pitched battles.
Punitive military raids and summary killings appear to be one response as Assad strives to keep control of Damascus and the northern commercial hub of Aleppo, opposition sources say.
Tanks and troops attacked the southwestern Damascus suburb of Mouadamiya on Monday and Tuesday, killing 86 people, half of them in cold blood, according to Assad's opponents.
It was hard to verify the assertion due to state curbs on independent media. Syrian leaders say they are fighting "armed terrorists" backed by Western and Gulf Arab nations out to topple Assad for his resistance to Israel and the United States.
Some foreign fighters from Arab and other countries have joined Syrian rebels, possibly including Rustam Gelayev, son of a late Chechen rebel warlord in Russia's Caucasus region.
Russian media and websites sympathetic to Islamist insurgents in the Caucasus reported that Gelayev had been killed in Syria, with some saying he had been fighting against Assad.
Russia's Kommersant daily, however, cited a relative of Gelayev as saying he had been studying in Syria, had decided to leave due to the violence and was killed on his way to Turkey.
In Aleppo, tanks shells crashed into buildings in the rebel-held Saif al-Dawla district, even as displaced civilians came back to check their houses or pick up abandoned belongings.
A man in a dirty T-shirt and tattered sandals, who gave his name as Mohammed, said his home was in the nearby neighbourhood of Salaheddine, now back in army hands after days of fighting.
"Me and my two brothers and our families left to stay with friends. I left with what I'm wearing. We are four families in one house," he said, as shells landed a few hundred metres (yards) away. "Does the world care about Syrians? I think not."
Aleppo, a once-prosperous city, is living through desperate times, divided by war, its streets stinking with uncollected rubbish and residents uncertain whether to flee or stay.
Rebel-held areas are at the mercy of army tanks, planes and helicopter gunships, with civilians now caught up in a conflict which Aleppo had mostly avoided until a rebel offensive in July.
"Where are we to go? Yesterday they hit the rebel base across the road, but nowhere is safe in Aleppo. The planes bomb everywhere," said a carpenter who feared to give his name.
"If there is a safe place in Syria, tell me. We don't have the money to leave the country," the 53-year-old added.
YouTube footage showed a funeral in Daraya of a mother and five children from the al-Sheikh family. Activists said the victims were killed by shellfire in the town after fleeing this week's military offensive on the Damascus suburb of Mouadamiya.
The bodies were wrapped in white shrouds, the children's faces exposed. Mourners laid green branches on the corpses and cried: "There is no god but Allah, Assad is the enemy of Allah."
International diplomacy has failed to brake the conflict in Syria, which the United Nations says has cost more than 18,000 lives since a popular uprising erupted in March 2011.
Outgoing U.N.-Arab League envoy Kofi Annan has blamed splits in the U.N. Security Council, where Russia and China have repeatedly blocked Western efforts to ramp up pressure on Assad, for the failure of his peace mission.
Babacar Gaye, the head of U.N. monitors sent to observe an abortive ceasefire declared by Annan on April 12, was expected to leave Damascus on Thursday. The mission's mandate has expired and was not renewed due to the spiralling violence.
Annan's successor, veteran Algerian diplomat Lakhdar Brahimi, was flying to New York for a week of consultations at the United Nations, his spokesman Ahmad Fawzi said.
For all Brahimi's skills, it is not clear how he can succeed where Annan failed, given the deadlock among big powers and the intractable conflict in Syria, where Assad's minority Alawite-based ruling system is pitted against mostly Sunni opponents.
The upheaval in Syria, at the heart of a volatile Middle East, is already spilling over into its neighbours.
Sporadic clashes between Sunnis and Alawites erupted for a fourth day in Lebanon's northern city of Tripoli, breaching a truce agreed less than 24 hours earlier, after Sunni gunmen shot dead an Alawite man. Nine people were wounded in the fighting.
At least 13 people have been killed and more than 100 wounded in Sunni-Alawite fighting in Lebanon this week that has been fuelled by sectarian tensions in Syria.
Ankara has grown alarmed at apparent links between Kurdish militants fighting in southeastern Turkey and the conflict in Syria. Prime Minister Tayyip Erdogan has accused Assad of backing Kurdistan Workers Party (PKK) fighters and says Turkey's military might act to counter any threat from the PKK in Syria.
Turkish and U.S. diplomats, intelligence and military officials held talks in Ankara on Thursday expected to touch on a possible buffer zone in Syria and steps to stop PKK militants in the border region from exploiting the chaos.