Forces loyal to President Bashar al Assad have killed at least 85 people in a "massacre", rebels fighting the regime have claimed.
Activists claim that some of those killed when troops stormed the Damascus suburb of Jdeidat al Fadel after five days were women and children.
Jamal al Golani, a member of the Revolution Leadership Council opposition group fighting Mr Assad's regime, said he had counted 98 bodies in the streets and 86 people who he said had been "summarily executed" in makeshift clinics where they were lying wounded.
He said: "Jdeidet al Fadel was militarily a lost cause from day one because it was surrounded by the army from every direction. There are almost no wounded because they were shot on the spot."
However, the rebels claims have not been independently verified.
The government troops stormed an area where there were said to be up to 270 rebels and it is feared the number of dead could be higher.
Abu Ahmad al Rabi', an activist in the adjacent district of Jdeidet Artouz, said: "We documented 85 summarily executed, including 28 shot in a makeshift hospital after Assad's forces entered Jdeidet al Fadel. We fear that the victims of the massacre are much higher."
The Syrian Observatory for Human Rights, an opposition monitoring group operating from London, said it documented 80 names of people killed, including three children, six women and 18 rebel fighters.
The reports come as the United States announced it is providing Syrian rebels with \$123m (£80m) in new non-lethal aid that may include body armour and other supplies.
The money will double the amount of non-lethal assistance to the Syrian opposition, as well as increase humanitarian aid.
The aid could include armoured vehicles, night vision goggles and advanced communications equipment.
Foreign ministers from the main supporters of the rebels trying to topple the Syrian government have been meeting in Istanbul to increase pressure on Syrian President Bashar al Assad.
The United Nations estimates the fighting in Syria has killed more than 70,000 people.
"The stakes in Syria couldn't be more clear," said US Secretary of State John Kerry.
"Chemical weapons, the slaughter of people by ballistic missiles and other weapons of huge destruction, the potential of a whole country ... being torn apart into enclaves, the potential of sectarian violence.
"This bloodshed needs to stop and that's what brought us here tonight on Saturday and a very early Sunday morning to talk about the possibilities for peace and transition."
In a joint statement, the foreign ministers said they welcomed the Syrian National Coalition's "firm rejection of extremism and its commitment not to use chemical weapons".
They cited those commitments in agreeing to enhance and expand support for all coalition institutions.
The foreign ministers said they recognised the "need to change the balance of power on the ground" and welcomed the additional pledges and commitments to further increase the support to the Supreme Military Council.
The head of the council provided a military briefing during the meeting.
Syrian opposition leader Moaz al Khatib said: "Our revolution is for the entire Syrian people."
US President Barack Obama has said he has no plans to send weapons or give lethal aid to the rebels, despite pressure from Congress, some administration advisers and appeals from the Syrian opposition leadership.
Since February, the US has shipped food and medical supplies directly to the Free Syrian Army, but Mr Obama recently expanded the aid to include defensive military equipment.
Mr Kerry's announcement on Saturday was the first under that new authorisation.
So far, the US has provided an estimated \$117m in non-lethal aid to the Syrian opposition.
Today, US Defence Secretary Chuck Hagel heads to Israel in a week-long trip to the Middle East that will be dominated by the Syrian conflict as well as the Iran's nuclear programme.