The 50-kilogram (110-pound) bomb, near a large hotel in a heavily guarded district, was described by state media as an attack by "terrorists" - the government's term for insurgents in the 19-month-old uprising against Assad.
Opposition activists said Sunday's blast appeared to be the work of the Ahfad al-Rasoul (Grandsons of the Prophet) Brigade, an Islamist militant unit that attacked military and intelligence targets several times in the last two months.
The mainly Sunni rebels have carried out a series of bombings targeting government and military buildings in Damascus this year, extending the war into the seat of Assad's power.
The Syrian conflict has aggravated divisions in the Islamic world, with Shi'ite Iran supporting Assad -- whose Alawite faith derives from Shi'ite Islam -- and U.S.-allied Sunni nations such as Saudi Arabia, Turkey and Qatar backing his foes.
The Syrian Network for Human Rights, an activist monitoring group, said government forces had killed 179 people on Sunday. It said most of the dead were civilians killed in shelling of Damascus suburbs and included 14 women and 20 children. The rest were rebels killed in battles in the capital and the northern provinces of Idlib and Aleppo.
Opposition campaigners said the Syrian army shelled rebel positions inside a Palestinian refugee camp on the edge of Damascus on Sunday, killing at least 20 people. They said the Yarmouk camp had become the latest battleground in the war.
In northern Idlib, opposition sources said rebels were forced to halt an offensive to take a big air base because of a shortage of ammunition, a problem that has dogged their campaign to cement a hold on the north by eliminating Assad's devastating edge in firepower.
Islamist insurgents had launched the attack on the Taftanaz military airport at dawn on Saturday, using rocket launchers and at least three tanks captured from the military.
The Syrian government restricts journalists' access in Syria, making it difficult to verify reports from the ground.
The Jaafar bin Tayyar Division, a rebel unit in Deir al-Zor, said its fighters had taken control of the al-Ward oilfield near the Iraqi border on Sunday, after overrunning a loyalist outpost that had 40 militiamen defending it.
Rebel commanders, former Syrian officials and the Syrian head of an oil services company familiar with oil production in the area said the fields, mostly not operational, had been under de facto rebel control for months.
Fears of wider conflagration
The conflict began with peaceful protest rallies that morphed into armed revolt when Assad, whose family has ruled Syria since 1971, tried to stamp them out with military might. About 32,000 people have been killed, wide swathes of the major Arab state have been wrecked and the civil war threatens to widen into a regional sectarian conflagration.
The opposition talks that began in Qatar marked the first concerted attempt to meld feuding, disparate groups based abroad and coordinate strategy with rebels fighting in Syria.
Divisions between Islamists and secularists as well as between those inside Syria and opposition figures based abroad have foiled prior attempts to forge a united opposition and deterred Western powers from intervening militarily.
Analysts were sceptical the planned four days of opposition talks in the Qatari capital Doha would bring immediate results.
They aim to broaden the Syrian National Council (SNC), the largest of the overseas-based opposition groups, from some 300 members to 400, to pave the way for talks in Doha on Thursday including other anti-Assad factions to crystallise a coalition.
"The main aim is to expand the council to include more of the social and political components. There will be new forces in the SNC," Abdulbaset Sieda, current leader of the Syrian National Council, told reporters in Doha ahead of the meeting.
The meetings would also elect a new executive committee and leader for the SNC, he said.
A Qatar-based security analyst, who asked not to be named, said the meetings would bring a small step forward, at most. "The Syrian National Council is just too divided," he said.
In Cairo, the international mediator on Syria, Lakhdar Brahimi, called on Sunday for world powers to issue a UN Security Council resolution based on a deal they reached in June to set up a transitional Syrian government.
But Russian Foreign Minister Sergei Lavrov, speaking at the same news conference, dismissed the need for a resolution and said others were stoking violence by backing rebels. His comments highlighted the impasse over Syria's civil war.
Russia and China, both permanent council members, have vetoed three Western-backed UN draft resolutions condemning Assad's government for the violence. The other three permanent members are the United States, Britain and France.