Reports are emerging that Syrian forces and militiamen loyal to President Bashar al-Assad killed at least 85 people, including women and children.
Forces stormed a Damascus suburb after five days of fighting, opposition activists in the area said.
An activist claims: "We documented 85 summarily executed, including 28 shot at a makeshift hospital after Assad's forces went in Jdeidet al-Fadel."
Syria's Sana state news agency said the military "inflicted big losses on terrorists in Jdeidet al-Fadel and destroyed weapons and ammunition and killed and wounded members of the terrorist groups".
Jamal al-Golani, a member of the Revolution Leadership Council opposition group, said the number of dead may be higher than 250.
He said that the victims were mostly shot at close range, but the presence of army patrols made documenting all of them difficult.
The killings happened over several days when pro-Assad forces stormed an area where there were up to 270 rebels.
Mr Golani said that 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 injured.
The working class district, part of Sunni Muslim towns surrounding the capital that have been at the forefront of the uprising against Assad.
It is situated near hilltop bases for elite loyalist forces, who mostly belong to Assad's minority Alawite sect, an offshoot of Shia Islam that has dominated the country since the 1960s.
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 Syrian Network for Human Rights said the International Committee for the Red Cross should be allowed to evacuate civilians from the district after credible reports of "extrajudicial killings and summary executions inside homes and tens of cases of sexual violence".
Earlier, The US said it is to double its non-lethal assistance to Syria's opposition in their two-year battle to oust President Bashar Assad's regime.
US Secretary of State John Kerry announced the new package of assistance in a written statement at the conclusion of a conference in Istanbul.
The additional aid brings total non-lethal US assistance to the opposition since the fighting began to $250m.
Mr Kerry said it highlighted the US support for a political solution to the crisis in Syria.
He said a portion of the new money would be used to follow through on US President Barack Obama's recent authorisation to expand direct supplies to the Free Syrian Army beyond food and medical supplies to include defensive items.
Mr Kerry said: "They (the Syrian National Coalition) are committed to a pluralistic Syria, to a Syria in which every minority has minority rights protected, that includes all of minorities in which everybody will have an ability to be able to make choices for the future.
"It is a vision that rejects terrorism and rejects extremism. It is a vision that is pledged to never use chemical weapons. It is a vision that is committed to a political solution."
Yet the pledge fell far short of what the opposition had made clear it wanted.
It had called for weapons and direct military intervention to stop the violence, which the UN says has killed more than 60,000 people.
The SNC had sought drone strikes on sites from which the regime has fired missiles.
It also seeks the imposition of no-fly zones and protected humanitarian corridors to ensure the safety of civilians.
Instead, the Obama administration has pledged to provide an additional $123m (€94.2m) in aid, which may include for the first time armoured vehicles, body armour, night-vision goggles and other defensive military supplies.
It was the only tangible, public offer of new international support as the foreign ministers of the 11 main countries supporting the opposition met in a marathon session in Istanbul.
Mr Kerry also announced nearly $25m (€19m) in additional food assistance for Syrians who remain inside the country.
Assistance will also go to those who have fled to neighbouring countries, bringing the total US humanitarian contribution to the crisis to more than $409m (€319m).
Mr Obama has said he has no plans to send weapons or give lethal aid to the rebels.
This is despite pressure from Congress, some administration advisers and the appeals from opposition.
European nations, meanwhile, are considering changes to an arms embargo that would allow weapons transfers to the Syrian opposition.
But European Union action is unlikely before the current embargo is set to expire in late May.