(Reuters) - A bomb exploded near army and security compounds in Damascus on Sunday, Syrian television said, and fractured opposition groups seeking to topple President Bashar al-Assad began unity talks abroad to win international respect and arms supplies.
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 the 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 terrorist explosion caused several injuries. One of (the injured) is in critical condition," state television said.
Video from state media and posted online by activists showed shattered windows and battered cars but little other damage.
Rebels have carried out a series of bombings targeting government and military buildings in Damascus this year, extending the civil war into the seat of Assad's power.
The Syrian Network for Human Rights, an activist monitoring group, said government forces had killed 57 people in bloodshed elsewhere in Syriaon Sunday. It said most of the dead were civilians and the rest rebels in battles that continued around Damascus and in the northern provinces of Idlib and Aleppo.
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, especially warplanes and helicopter gunships.
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, disrupting government air strikes on nearby rebel-held towns and villages.
FEARS OF WIDER CONFLAGRATION
The conflict began with peaceful protest rallies that morphed into armed revolt when Assad 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 offering more than moral support.
Four days of talks in the Qatari capital Doha were expected with the goal of overhauling and broadening the Syrian National Council (SNC), the largest of the overseas-based opposition groups, from some 300 members to 400.
SNC leaders hoped this would pave the way for a separate meeting in Doha on Thursday including other anti-Assad factions to crystallize a united 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.
The United States called last week for an overhaul of the opposition's leadership, saying it was time to move beyond the SNC and bring in those "in the front lines fighting and dying".
The mainly Sunni Muslim rebels are supported by Sunni states including Saudi Arabia, Qatar and neighboring Turkey. Shi'ite Iran remains the staunchest regional supporter of Assad, whose Alawite faith derives from Shi'ite Islam.
Veteran opposition leader Riad Seif has proposed a structure melding the rebel Free Syrian Army, regional military councils and other insurgent units alongside local civilian bodies and prominent opposition figures.
Called the Syrian National Initiative, his plan envisages the creation of an Initiative Body, including political groups, local councils, national figures and rebel forces; a Supreme Military Council; a Judicial Committee and a transitional government-in-waiting composed of technocrats.
Western, Turkish and Arab recognition of the new opposition structure, Seif said in an interview with Reuters last week, will help channel anti-tank and anti-aircraft missiles to the rebels and "decide the battle".
Western powers keen to see Assad removed have been loath to arm rebels in part because of the growing prominence of radical Islamic fighters fighting separately from more secular groups.
(Additional reporting by Rania el Gamal and Regan Doherty in Qatar, Suleiman al-Khalidi in Amman; Editing by Mark Heinrich)