Two loud explosions targeted one of Syria's top military command buildings in the capital Damascus on Wednesday, engulfing the building in flames, residents and state-television said.
They said the explosions struck the General Staff Command Building (Hay'at al Arkan) in the Umayad Square in central Damascus, which is one of the top military headquarters in the country.
There was no immediate word on casualties but ambulances could be heard racing to the sealed-off area.
The rebel Free Syrian Army claimed responsibility for twin bombings.
"The Free Army hit the General Staff building in Damascus' Umayyad Square and dozens were killed in the two powerful blasts," the information office for the FSA military council said in a statement. Syria's Information Minister Omran Zoabi had earlier said that the blasts caused only material damage.
Gunfire and other smaller blasts could be heard after the explosions, as well as the sound of ambulance sirens. Many roads in the center of the capital were blocked, residents said.
The explosions were heard around 7.00 am (11.45 p.m. EDT on Tuesday), before regular working hours start in Syria.
"The explosions were very loud. They shook the whole city and the windows of our house were shuddering," one resident reached by telephone said.
"Black smoke was seen rising from the area near the army staff building," the resident, who declined to be named, said.
A Damascus bomb attack on July 18 killed several top security officials, including Assad's brother-in-law, the defense and interior ministers. That attack paved the way for a rebel advance into the center of the capital, although they have since been pushed back to the outskirts.
Another resident said: "I was woken up at four minutes to seven by the first loud explosion. Five or six minutes later there was a second."
"We're used to the sound of artillery but these were very big - bigger than usual. I can hear gunfire still," he said, speaking an hour and a half after the blasts.
He said one of the blasts appeared to have been in the area of the General Staff Command.
He said he could see soldiers stationed on the roof of the nearby Air Force Intelligence building.
Syria's conflict, once a peaceful protest movement, has evolved into a civil war that the UN special envoy to Syria, Lakhdar Brahimi, said was "extremely bad and getting worse." He said the stalemate in the country could soon "find an opening", without elaborating.
Even Damascus has become a battleground between Assad's forces and opposition fighters.
Activists say more than 27,000 people have been killed in the 18-month-old uprising against Assad.
With no foreseeable prospect of foreign intervention and diplomacy stuck, outgunned rebels have relied increasingly on attacks with homemade bombs, striving to level the playing field against a state using fighter jets, artillery and tanks.
At the annual UN General Assembly in New York, French President Francois Hollande sought to shake up international inertia over Syria's crisis by calling for UN protection of rebel-held areas to help end Syria's bloodshed and rights.
"The Syrian regime ... has no future among us," Hollande said in a speech. "Without any delay, I call upon the United Nations to provide immediately to the Syrian people all the support it asks of us and to protect liberated zones."
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