Britain, meanwhile, said it would refer the alleged chemical weapons attack, which could not immediately be verified and has been vehemently denied by the Damascus regime, to the United Nations.
While the Syrian National Coalition gave a toll of 650 dead, a Britain-based monitoring group said earlier that at least 100 people had been killed and the number was rising.
"This figure will surely go up. The raids and bombardment are continuing," said the Syrian Observatory for Human Rights, which gathers its information from activists and medics.
It did not comment on allegations by anti-regime activists that the army had used chemical arms in its bombardment of the densely-populated Ghouta suburbs where rebels have been holding out against government forces.
According to activists, hundreds of people died of gas inhalation and exposure to chemical weapons.
Syrian authorities, for their part, denied charges that the army used chemical arms.
"Reports on the use of chemical weapons in (the suburbs of) Ghouta are totally false," state news agency SANA said.
"It's an attempt to prevent the UN commission of inquiry from carrying out its mission."
The Syrian National Coalition called for an urgent UN Security Council meeting. "I call on the Security Council to convene urgently," National Coalition leader Ahmed al-Jarba told Al-Arabiya news channel, condemning the bombardment of the Damascus suburbs as a massacre.
British Foreign Secretary William Hague, meanwhile, said his country will refer the opposition charges of a massive chemical weapons strike to the Security Council.