Syrian state television said a "terrorist explosion" which struck a national security building in Damascus on Wednesday occurred as government ministers were meeting officials.
"The terrorist explosion which targeted the national security building in Damascus occurred during a meeting of ministers and a number of heads of (security) agencies," the television.
Activists at the scene said that Republican Guard troops sealed off Damascus's Shami Hospital after ambulances arrived with casualties from the explosion.
A report from Lebanese-based news channel Al-Mayadeen said that several senior security officials were killed in the explosion.
Overnight, the revolt against Syrian President Bashar Assad came within sight of his presidential palace as fighting erupted in major Damascus neighborhoods for a fourth day.
An army barracks near the "palace of the people," a huge Soviet style complex overlooking the sprawling capital from the western district of Dummar, came under rebel fire around 7.30 a.m. (0430 GMT), activists and a resident said.
"I could hear the sound of small arms fire and explosions are getting louder and louder from the direction of the barracks," Yasmine, who works as an architect, said by phone from Dummar.
Video footage broadcast by activists purportedly showed fire in the barracks overnight as a result of an attack by mortar rounds, but residents who saw the fire said they had not heard explosions to indicate it was a result of an attack.
Dummar is a secure area containing many auxiliary installations for the presidential palace and the barracks is just hundreds of meters from the palace itself.
Fighting also erupted overnight in the southern neighborhoods of Asali and Qadam, and Hajar al-Aswad and Tadamun - mainly Sunni Muslim districts housing Damascenes and Palestinian refugees.
Assad and the ruling elite belong to the minority Alawite sect, an offshoot of Shi'ite Islam that has dominated power in Syria since a 1963 coup.
Government troops used heavy machine guns and anti-aircraft guns against rebels moving deep in residential neighborhoods, armed mostly with small arms and rocket propelled grenades.Opposition is cautious
Rebels directed their fire overnight at a large state facility turned headquarters for pro-Assad militia, known as shabbiha, drawn mainly from Alawite enclaves in nearby hills.
Army tanks and anti-aircraft guns, used as an infantry weapon, took positions in the northern neighborhood of Barzeh, where hundreds of families from the neighboring district of Qaboun are seeking shelter.
"Anti-aircraft guns are firing at Qaboun from Barzeh. There are lots of families in the streets with no place to stay. They came from Qaboun and from the outskirts of Barzeh," said Bassem, one of the activists, speaking by phone from Barzeh.
In the central neighborhood of Midan tanks and infantry fighting vehicles known as BDMs took positions in main thoroughfares and sporadic fighting was reported.
"Armour have not been able to enter the alleyways and old streets of Midan. The neighborhoods old Zahra and the old area near Majed mosque are in the hands of the rebels," said Abu Mazen, an activist in the district.
Rebel fighters have called the intensified guerrilla attacks in recent days, which have targeted shabbiha buses, unmarked intelligence patrols and armored vehicles in the capital, the battle "for the liberation of Damascus" after 16 months of revolt.
But senior opposition figures took a more nuanced view.
"It is going to be difficult to sustain supply lines and the rebels may have to make a tactical withdrawal at one point, like they did in other cities," veteran opposition activist Fawaz Tello said from Istanbul.
"But what is clear is that Damascus has joined the revolt," Tello, a Damascene, told Reuters. "By hitting well known Sunni districts of the city, such as Midan, the regime is exposing the sectarian nature of the crackdown."
Information Minister Omran Zoabi said on Tuesday security forces were fighting armed infiltrators in Damascus. He said many had surrendered while others "escaped on foot and by car and are firing randomly in the air to frighten people."