Syrian opposition activists say scores of bodies have been found in a town near the capital Damascus, accusing government troops of a "massacre".
The activists say many of the victims in the town of Darayya had been "summarily executed".
According to unconfirmed reports, 200 bodies were discovered in houses and basement shelters.
Without commenting on the activists' claim, Syrian state TV said Darayya was being "cleansed of terrorist remnants".
Meanwhile, Syrian Vice-President Farouq al-Shara has greeted an Iranian delegation in Damascus, quashing weeks of speculation that he had defected to the opposition.
The forces of President Bashar al-Assad launched an assault on Darayya on Saturday, after days of heavy bombardment.
The BBC's Barbara Plett in Beirut says the attack was part of a wider campaign to reclaim the southern outskirts of Damascus, where rebels have been regrouping since being driven out a month ago.
Activists on the ground later posted unverified video footage on the internet, which shows rows of bodies side by side in the Abu Auleiman al-Darani mosque.
The activists say that many of the victims had gunshot wounds to the head and chest and were killed during house-to-house raids by government troops.
"Assad's army has committed a massacre in Darayya," an opposition member told Reuters.
The activist added that most of the victims had been killed at close range, and some died from sniper fire.
The Syrian Observatory for Human Rights, a UK-based opposition activist group, said that altogether 183 people died across Syria on Saturday.
The claims by the activists have not been independently verified because of restrictions placed on foreign media across Syria.
Local activists say the type of mass killing reportedly carried out in Darayya, with dozens of bodies being discovered following government raids, has increased in recent months.
Human Rights Watch said this is not a new pattern, but is now happening in more areas and in greater numbers.
An earlier report from United Nations observers found that both sides had carried out massacres, but the Syrian army was responsible for a far greater number of deaths.
In a separate development, the head of the UN mission to Syria left the country after the mission had been wound up.
Senegalese Lt Gen Babacar Gaye joined a UN convoy to Lebanon on Saturday.
Last week, the UN decided against extending the mission, which was originally part of a six-point peace plan for Syria.
However, the ceasefire mandated by the plan never took hold and rising violence forced the UN monitors to be confined to their hotels since June.