An explosion Sunday tore apart a bus carrying Shiite pilgrims in a central neighborhood of the Syrian capital Damascus, according to official and opposition accounts.
The official Syrian state news agency, SANA, reported six dead and more than a dozen wounded as a result of the blast in the city's Kalaasa area, close to the famed Hamidiyeh market. The British-based, pro-opposition Syrian Observatory for Human Rights confirmed the attack but put the death toll at seven.
The bustling shopping zone has been relatively untouched by the war that has pummeled Syria for almost four years. Soldiers and police keep a close guard on the capital, base of President Bashar Assad.
Quoting an unidentified police source, SANA reported that "terrorists" -- the official designation for anti-government rebels -- placed an 11-pound explosive canister in front of the bus, "which led to the martyrdom of six people and the wounding of 19 others." A second bomb of similar size was defused before it could detonate, the news agency reported.
The Nusra Front, an Al Qaeda-affiliated rebel group, claimed responsibility for the attack in a statement on its official Twitter account. It said the attack was carried out by a suicide bomber, an assertion at odds with the official account.
"One of the heroes of the Nusra Front was able to delve into a bus ... where he detonated his explosive belt," said the statement, adding that the operation was an act of vengeance on behalf of Sunni Muslims in Syria and neighboring Lebanon.
The bus, which had Lebanese license plates, was carrying Shiite pilgrims from Lebanon, according to various accounts.
Shiites flock to Damascus to visit the Sayeda Zainab shrine, an important religious landmark southeast of the capital. The shrine itself has periodically come under attack from Syrian rebels, who are largely Sunni Muslims, the majority sect in Syria. Some Sunni Islamist rebels in Syria view Shiites as apostates and government supporters.
Damascus is a frequent target of rocket strikes and shelling from rebels based in outlying neighborhoods. But bombings such as Sunday's attack have been relatively infrequent in recent months amid the capital's tight security and myriad checkpoints.