Hillary Clinton seeks Syria action from 'paralysed' UN
Hillary Clinton has called on the United Nations Security Council to make a renewed effort to deal with the conflict in Syria.
The US secretary of state said the council was "paralysed".
China and Russia have vetoed two attempts by Western members to impose tougher measures on the regime.
UK-based activist group the Syrian Observatory for Human Rights said more than 305 people died on Wednesday, the bloodiest day of the conflict so far.
The observatory's Rami Abdel Rahman said the figure included only those whose names had been documented.
"If we count the unidentified bodies, the figure will be much higher," he said.
The observatory (SOHR) said 199 of Wednesday's dead were civilians.
The SOHR is one of the most prominent organisations documenting and reporting incidents and casualties in the Syrian conflict. The group says its reports are impartial, though its information cannot be independently verified.
Other groups estimated Wednesday's death toll to be considerably higher.
Most of the deaths were in and around the capital Damascus, where activists accused pro-government militiamen of murdering women and children in their homes.
The observatory also said 40 bodies were found in the town of al-Dhiyabiyeh near Damascus.
The deaths came on the same day that the military's main headquarters was badly damaged in an apparent suicide car-bombing.
'Reign of terror'
France, Britain and the United States have argued for months in favour of taking stronger action against Syrian President Bashar al-Assad.
But Russia and China have blocked their proposed resolutions.
Mrs Clinton said that the Security Council must end the violence and urged the members to "try once again to find a path forward".
The UK's David Cameron also called on the UN to act, telling the annual meeting of the UN General Assembly that recent evidence of crimes against children in Syria is "a terrible stain on the reputation of this United Nations".
He singled out those countries that "failed to stand up to these atrocities and in some cases aided and abetted Assad's reign of terror" for particular criticism.
The five permanent members of the council, who all hold vetoes, have so far been unable to agree on a course of action regarding the conflict in Syria, which has claimed some 27,000 lives over the last 18 months.
In Damascus, the rebel Free Syrian Army (FSA) and an Islamist group called Ansar al-Islam both said they carried out the attack on the military headquarters.
State TV broadcast footage of a minibus slowing before exploding at the military compound near one of the capital's main road junctions.
Gunfire reverberated around the city for hours after the bombings, as rebels fought with soldiers at the compound.
The FSA announced earlier that it had moved its command from Turkey to Syria in an apparent attempt to bolster its fight against regime forces.
By Lyse Doucet inside Syria
Security measures were stepped up around the military compound after a rebel attack in July killed several senior security officials, including the defence minister and President Bashar al-Assad's brother-in-law.
Although the attack is a spectacular coup, the rebels do not seem to have caused high-level military casualties, or to have taken and held ground, the BBC's Jim Muir in Beirut reports.
And they have diverted attention from terrible carnage elsewhere, including in nearby suburbs, where activists say dozens of people have been summarily executed by regime forces.