Two bombs struck the Syrian army headquarters in the heart of Damascus, as US Secretary of State Hillary Clinton appealed for the "paralyzed" UN Security Council to find a way to end the conflict.
The attack on the heavily guarded site killed four guards and sparked a gun battle between troops and rebels, Syrian state media said.
A rebel officer and the Syrian Observatory for Human Rights said it was an inside job, while an Islamist rebel group said its men carried out the attack, and five of its fighters including a suicide bomber died during the assault. The claims were impossible to verify.
According to the army, "terrorist groups" with foreign affiliations "carried out a new act of terrorism by detonating a car bomb and another device on the edge of the general staff compound."
The regime of President Bashar al-Assad blames unrest and violence on foreign-backed terrorist gangs.
All senior commanders and other officers were safe and uninjured, the statement read.
State television showed video footage of a white van exploding beside the military headquarters, and a second blast inside the compound. It said the bombings came 10 minutes apart, and that 14 people were wounded.
A spokesman for the Free Syrian Army's Military Council in Damascus, Ahmed al-Khatib, said the attack was staged with two car bombs, but denied they were driven by suicide attackers.
Syria's military said the "terrorist explosions around and inside the army headquarters were caused by two car bombs driven by suicide attackers."
It was the biggest attack on the security apparatus since a July 18 suicide bombing against a heavily guarded headquarters in Damascus killed four top regime officials, including defense minister General Daoud Rajha and Assad's brother-in-law, Assef Shawkat.
The strike came as the bloodshed, which left as many as 217 dead on Tuesday, dominated proceedings at the UN General Assembly in New York.
"The atrocities mount while the Security Council remains paralyzed and I would urge that we try once again to find a path forward" so that the council can try to end the violence, Clinton said.
Her appeal came amid mounting attempts by Western nations to press Russia and China to ease their opposition to UN action against the Assad regime.
Russia, Syria's key ally, and China have used their permanent member security council veto to thwart resolutions that could have led to economic sanctions on Syria.
British Prime Minister David Cameron said the blood of children killed in the conflict had become "a terrible stain on the reputation of this United Nations," and blasted "those who have failed to stand up to these atrocities and in some cases aided and abetted Assad's reign of terror."
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Arab ministers also weighed calls for an Arab intervention in Syria, and met with UN-Arab League envoy Lakhdar Brahimi on the General Assembly sidelines.
Tunisian President Moncef Marzouki told AFP that his country could support an Arab peacekeeping force, and called Assad "a bloodthirsty dictator."
"We have really pushed for a peaceful solution, but if it is necessary, it must be an Arab peacekeeping force, yes," he said.
The emir of Qatar, Sheikh Hamad bin Khalifa Al-Thani, called at the UN General Assembly for an Arab intervention in Syria on Tuesday. However Arab League secretary general Nabil al-Arabi told reporters he did not believe the emir intended it to be a "fighting force."
Western diplomats say they do not expect Russia to weaken its defense of Assad. But China, which does not have the same strategic interests, may be feeling pressure from Arab and other nations over its position.
Russian Foreign Minister Sergei Lavrov showed no sign of changing position Wednesday, and said violence by the government and opposition had to be condemned.
Separately, Iranian President Mahmoud Ahmadinejad told reporters that Tehran was working to set up a contact group on the conflict in Syria, in a move unlikely to be welcomed by many nations.
He refused to say which states had been approached to join, saying he was hopeful his foreign ministry would soon be making an announcement.
Ahmadinejad also accused "outside forces" of meddling in Syria, without directly naming any countries.
Aid groups in Syria need better access to people ravaged by the war, said European Union humanitarian aid commissioner Kristalina Georgieva.
"With winter approaching, more populations are at risk," Georgieva said after meeting with officials from NGOs and the main countries providing aid to Syria on the sidelines of the General Assembly.
The UN humanitarian affairs chief, Valerie Amos, said that 2.5 million people affected directly or indirectly by the war need aid.
Some 1.2 million people have been displaced inside Syria and another 300,000 have sought refuge in neighboring countries, such as Turkey and Lebanon, exerting tremendous pressure on them.
In Beirut, the Syrian Observatory for Human Rights reported that more than 30,000 people, most of them civilians, have been killed in violence since the outbreak of a revolt against Assad in March 2011.
The toll does not include thousands of people arbitrarily detained and held in Syrian jails, he added, nor does it include hundreds of unidentified corpses, most of which were found across the war-torn country in the past three months.
On Wednesday alone, the death toll rose to 343, the highest daily toll since the unrest began, a spokeswoman for the Local Coordination Committees of Syria told CNN.