Damaged buildings in the old city of Homs
Syrian government forces launched two operations in Damascus to root out rebel activists on Wednesday, killing at least 70, the opposition has said.
Troops reportedly went from house to house demanding to see people's papers, and summarily executed many of their victims, according to activists.
Syrian state TV said "dozens of terrorists" surrendered or were killed in the operation.
Meanwhile rebels say they are attacking an airport near the city of Aleppo.
Opposition forces are reportedly using a tank captured from the army to bombard the Menagh air base near the northern city.
Heavy clashes were also reported near the border with Turkey, about 50km (30 miles) from Aleppo.
Witnesses and activists described heavy shelling and a ground assault on Damascus's south-western suburb of Jdeidet Artouz.
One resident told Reuters news agency the soldiers had inspected his ID, and then let him go.
He later saw bodies of at least 35 men.
"Almost all of them were executed with bullets to their face, head and neck," said the man, who identified himself as Fares.
The UK-based Syrian Observatory for Human Rights said pro-Assad forces arrested about 100 people and tortured them.
"On Thursday morning after the operation the bodies of 43 people were recovered. Some of them had been summarily executed," the organisation said in a statement.
Other activist groups gave higher figures for the number of deaths.
An army spokesman told state TV that a number of armed men had clashed with government forces who had raided a farm.
Another regime assault on the southern Damascus suburb of Yalda also resulted in the deaths of at least 27 people, activists said, without providing any further details.
Meanwhile, the fighting in Aleppo appears to have settled into a stalemate.
The BBC's Jim Muir in neighbouring Lebanon says state TV had initially forecast that the city would be cleared of rebel fighters in just a few days.
But it is now reporting only a few incidents where "terrorist mercenaries" had suffered heavy losses.
Activists estimate some 20,000 people have died since anti-government protests erupted in March last year.