The rebel Free Syrian Army (FSA) claimed responsibility for twin explosions targeting the government's military headquarters in central Damascus on Wednesday, DPA reported.
There were conflicting reports on casualties.
The key rebel group, which recently moved its command into Syria from Turkey, said that dozens were killed in the blasts.
But shortly after the attack, Syrian Information Minister Omran al-Zogabi said the blasts had caused "material damage," but no casualties.
The army said all commanders and officers inside the building, on Ummayad Square, were unhurt in the assault.
However, official Syrian television, citing a military source, said four guards were killed and 14 people, including civilians, injured.
The broadcaster said preliminary investigations indicated that the explosions had been carried out by suicide bombers driving cars, contradicting an earlier account by al-Zogabi that they were caused by two roadside bombs.
The bombings came days after the FSA, comprising mainly defectors from the regular Syrian army, announced it had moved its command centre from Turkey to Syria, vowing to launch an offensive inside the capital.
Damascus has been hit by a series of bombings in recent months, which the government has blamed on "armed terrorists groups."
The most violent assault was on July 18, when rebels bombed a government security building, killing four key officials, including the defence minister and President Bashar al-Assad's brother-in-law.
Meanwhile, Iran's English-language television station, Press TV, said its reporter in Damascus was killed and the station's bureau chief injured while reporting from the scene of the twin explosions.
"Press TV correspondent Maya Nasser has been killed by sniper fire in Damascus," the broadcaster said on its website.
Hussein Mortada, a Lebanese national, who heads the office of Press TV and another Iranian network, Al-Alam, was shot in the back, it added.
Press TV, and Al-Alam, an Arabic-language network, are financed by the Iranian government, which is a close ally of al-Assad.
A total of 280 people were killed Wednesday across Syria. Activists posted late Wednesday pictures from what they claimed to be a massacre in Dhiyabia, near Damascus, during which they said at least 107 people were killed. Earlier activists had said 40 people were massacred in Dhiyabia.
"The bodies are in the dozens. Look, Muslims, look what this dictator (Bashar al-Assad ) is doing," said a man in the video footage.
Some of the footage showed men shot in the forehead, face or neck.
Activists said 13 civilians were also summarily executed in the district of al-Bayadia in the flashpoint province of Homs in central Syria, according to the London-based Syrian Observatory for Human Rights.
The bulk of the casualties were in the embattled northern city of Aleppo and the eastern province of Deir al-Zour, it said.
News from the country cannot be independently verified, as authorities bar most foreign media from restive areas in the country.
Meanwhile, the London based Syrian Observatory for Human Rights said the death toll since Syria' uprising started in March 2011 has 30,000 most of whom were civilians.
In neighbouring Lebanon, a Lebanese Shiite pilgrim, Awad Ibrahim, who was kidnapped in Syria in May returned home a day after he was released by his abductors, the National News Agency reported.
Eleven Lebanese Shiite pilgrims were kidnapped on May 22 in the northern province of Aleppo, while returning from a pilgrimage in Iran.
The hostage-takers, who claimed to be from the Free Syrian Army, have called on the leader of the Shiite Movement Hezbollah to apologize to the Syrian people for his continued backing of the Syrian regime.