Syrian opposition activists have accused President Bashar al-Assad's army of massacring hundreds of people in a town close to the capital that government forces recaptured from rebels.
In the town of Daraya, southwest of Damascus, some 320 bodies, including women and children, were found in houses and basements, according to activists who said most had been killed "execution-style" by troops in house-to-house raids.
Activists uploaded several videos to the Internet showing rows of bloodied bodies wrapped in sheets.
Most of the dead appeared to be young men of fighting age, but at least one video showed several children who appeared to have been shot in the head. The body of one toddler was soaked in blood.
Due to restrictions on non-state media in Syria, it was impossible to independently verify the accounts.
Clashes are raging across Syria as the 17-month-old rebellion grows increasingly bloody, particularly in the northern city of Aleppo, where the army and rebels appear stuck in a war of attrition.
Fighting in Aleppo on Sunday was the heaviest in the past week, according to Reuters journalists on the ground.
Fighter jets dropped bombs and fired missiles on rebel-held districts in the south of Aleppo, Syria's largest city, as residents fled in panic.
Reuters journalists there heard heavy explosions as clouds of black smoke rose into the air.
The uprising, which began as peaceful protests, has become a brutal civil war.
United Nations investigators have accused both sides of war crimes, but laid more blame on government troops and pro-government militia than on the rebels.
Mr Assad, who met an Iranian parliamentary delegation in Damascus, said the crisis was the result of Western and regional states trying to crush Syria's role in the "resistance" against Western and Israeli domination in the region.
"What is happening right now is not just a plot directed against Syria but the region as a whole, of which Syria is a foundational stone," he was cited as saying by state news agency SANA.
"The Syrian people will not allow this plan to reach its goals, no matter the cost."
The UN estimates that more than 18,000 people have been killed in the conflict.
Diplomatic efforts to stop the violence in Syria are stalled by a stalemate dividing Western countries, Gulf Arab states and Turkey, who all support the opposition, and Iran, Russia and China, who support Mr Assad.
With veto-wielding Russia leading resistance to action against Mr Assad, the UN Security Council remains deadlocked.
Egypt is seeking to arrange a four-way meeting with Turkey, Saudi Arabia and Iran, the main regional players in the conflict.
Iran, a Shia power, is Mr Assad's main backer, while Saudi Arabia is believed to be supplying weapons to the rebels.
Iran accuses its foes in the West and the Arab world of fuelling the conflict by arming the opposition. Deputy Foreign Minister Hossein Amir-Abdollahian welcomed Egypt's initiative.
"Iran has its own views about the political process in Syria and will put forth these views upon the formation of this committee," he said.
"We see any foreign intervention, terrorist actions and armed movements against the wishes of the people of Syria."
The Iranian parliamentary delegation that met with Mr Assad also visited Vice-President Farouq al-Sharaa.
It was Mr Sharaa's first public appearance in weeks, quashing activist rumours that he had defected to the opposition.
Alaeddin Boroujerdi, head of the delegation, said Iran had also suffered from "terrorism" and would continue to support the Syrian government.
"Just as Iran suffered from terrorism and overcame that difficult period, Syria will be able to do so as well," state news agency SANA quoted him as saying.
"Syria's security is Iran's security."