A blast ripped through a bus carrying Lebanese Shiite pilgrims in Damascus on Sunday, killing at least nine people, a monitor said, in an attack claimed by al-Qaida's Syrian branch.

The Syrian Observatory for Human Rights said at least 20 people were wounded in the explosion near Souq al-Hamadiyeh district, and that six of the dead were Lebanese citizens.

In Beirut, the Oshaq al-Hussein pilgrimage agency that organized the trip said all the passengers on the bus were Lebanese, identifying the dead as Mohammed Ahmed Meqdad, Mahdi Youssef Meqdad, Qassem Hatoum, Ali Abbas Ballouq, Shadi Houmani and Mohammed Hassan Ayyoub.

"They set out from Beirut at 5:30 am (0330 GMT) this morning," agency employee Fadi Khaireddin told Agence France-Presse, adding that the bus had space for 52 pilgrims, as well as the driver and trip administrator.

"The bus is usually full," he added, though he could not confirm how many people were on the trip this weekend.

He said the bus had made its first stop at the Sayyida Roqaya shrine and was heading to the revered Sayyida Zeinab shrine in southeast Damascus when the attack occurred.

Khaireddin said the group had been making regular trips throughout the Syrian conflict, with groups leaving each weekend for a day-long visit to shrines revered by Shiite Muslims across the border.

Al-Nusra Front, the affiliate of al-Qaida in war-ravaged Syria, claimed responsibility for the attack in a statement posted online.

Syrian state media, which reported a toll of six dead and 19 wounded, said the blast was caused by an explosive device rather than a suicide bomber.

State news agency SANA said officials had found and defused a second bomb that had been placed inside the bus before it detonated.

Observatory director Rami Abdel Rahman said the bus had a Lebanese license plate and was carrying Shiite Muslim pilgrims visiting religious sites in Damascus.

Syrian state television showed footage from the scene of the blast, with men in military uniforms picking through the wreckage of the bus.

Its front half was mostly blown off, leaving only the metal frame, and bags of belongings were strewn across the remaining seats.

The channel also showed images from inside a hospital where the wounded were being treated, including a woman whose black robes had been lifted up, revealing a bloodsoaked undershirt.

Meanwhile, Hizbullah, which has sent scores of fighters to aid the Syrian regime against the Islamist-led uprising, issued a statement denouncing the attack.

This "is part of the series of explosions that targets pilgrims in Syria, civilians in Iraq, believers in Pakistan" and "proves the barbarity of the terrorists," it said in a statement.

“Those who carried out the Damascus bombing are serving the interests of the Zionist entity and the scheme seeking to fragment the region,” the party added.

Parts of Damascus have remained relatively unscathed by the fighting raging across much of Syria since an uprising erupted in March 2011.

But rebels regularly fire rockets into the capital from rear bases in the surrounding countryside, and the city has also been hit by bombings.

Despite the conflict, the road from the Lebanese border to Damascus remains relatively safe, and Lebanese Shiite pilgrims have continued to visit religious sites in Syria.

More than 200,000 people have been killed in Syria since the conflict started, and around half of the country's population has been displaced.