Two suicide bombs have struck the heavily guarded Syrian army headquarters in the heart of Damascus, killing four guards and sparking a gun battle between troops and rebels, state media says.
A rebel officer and a rights group said the audacious attack which also left 14 people wounded was an inside job, while an Islamist rebel group said five of its fighters including a suicide bomber died in carrying out the assault.
Iran's Press TV meanwhile said one of its correspondents, a Syrian national, was killed by sniper fire and its Damascus bureau chief wounded as they reported from the scene.
The spectacular assault on the army's operations command centre came as the worsening bloodshed, which left at least 217 dead on Tuesday, dominated proceedings at the UN General Assembly in New York.
"Armed terrorist groups with affiliations abroad this morning carried out a new act of terrorism by detonating a car bomb and another device on the edge of the general staff compound," an army statement said.
"All senior commanders and other officers are safe and sound, and none of them was wounded," the statement carried by state media said.
But state television, citing a military official, said four troops guarding the headquarters were killed, and 14 civilians and soldiers injured.
The broadcaster showed video footage of a white van exploding on the roadside next to the building housing the headquarters, and a second blast inside the compound. It said the bombings were 10 minutes apart.
"The initial investigation shows that these terrorist explosions around and inside the army headquarters were caused by two car bombs driven by suicide attackers," the military official said.
President Bashar al-Assad's regime has systematically blamed unrest and violence on foreign-backed terrorist gangs ever since the revolt erupted in March 2011.
The rebel Free Syrian Army's Military Council in Damascus said on its Facebook page that "the Free Syrian Army has struck the military headquarters in Damascus's Umayyad Square."
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. The claims were impossible to verify.
"The operation was carried out by several Free Syrian Army battalions working with an officer and his troops on the inside," said Ahmed al-Khatib, spokesman for the FSA's Military Council in Damascus.
Khatib confirmed the attack was staged with two car bombs, but denied they were driven by suicide attackers, which is not a tactic used by the FSA in the past.
"A few minutes after the attack, fighters broke into the HQ compound with machineguns and rocket-propelled grenades," said Khatib.
An Islamist rebel group, Tajamo Ansar al-Islam (Gathering of Partisans of Islam) also claimed responsibility. Five of its fighters, including a suicide bomber, were killed in the attack, the group said in a statement posted online.
Early on Wednesday, pro-government militia executed at least 16 civilians in their homes in the Barzeh neighbourhood of north Damascus, according to the Observatory.
"Militiamen burst into their homes in the Barzeh neighbourhood at 5 am (1200 AESMT) and shot them dead," Observatory head Rami Abdel Rahman said, adding the dead included six women and three children.
Barzeh is a Sunni Muslim district considered sympathetic to the opposition to Assad's minority Alawite-dominated regime.
The opposition's foreign backers ramped up calls for Assad's exit at the General Assembly in New York, while the Observatory said that more than 30,000 people, mostly civilians, had been killed in the conflict so far.
Qatar's emir, Sheikh Hamad bin Khalifa Al-Thani - whose government is, with Saudi Arabia, the leading champion of arming the rebels - called on Tuesday for Arab military intervention in the face of the failure of efforts at the UN Security Council to end the bloodshed.
UN Secretary General Ban Ki-moon called on the Security Council to "solidly and concretely" support the peace efforts of UN-Arab League peace envoy Lakhdar Brahimi, who has stated there will be no quick solution.