In a comment posted on Twitter, Makdisi said that the blast was carried out using 500 kilograms (1,100 pounds) of explosives.

UN military observers tasked with monitoring a shaky truce in Syria have visited the site of the bombing, Makdisi added.

The group said the bomb went off on a street where a military hospital and air force intelligenceoffices are also located.

There was no immediate independent verification of the accounts from Syria, which has limited journalist access during a 14-month old uprising against President Bashar al-Assad.

The uprising, which began with mass protests Assad's forces sought to crush, now features an armed insurgency which the Syrian government describes as a "terrorist" network funded and directed from abroad.

Twin suicide bombers who struck near an intelligence outpost in Damascus on May 10 killed at least 55 people in one of the largest attacks to target government installations.

Syrian officials call those blasts proof of a "terrorist" campaign against the state, while Assad's opponents accuse him of staging the attacks to validate those claims and justify a bloody crackdown on largely peaceful dissidents.

Syria is five weeks into a peace plan sponsored by the United Nations and Arab League including a U.N. monitoring mission intended to oversee a ceasefire - yet to take hold - aimed at paving the way for a political path out of bloodshed. REUTERS