A bomb exploded in central Damascus near the hotel used by the UN observer mission in Syria on Wednesday, wounding three people, Syrian state television reported.
It said the blast, in the Abu Remanih area near a military depot and the trade union headquarters, was caused by a bomb planted on a fuel truck.
Several ambulances rushed to the scene and a security cordon was set up, according to witnesses.
Activists said massive explosions echoed across much of the city and a video released by one opposition group showed a vast plume of smoke billowing into the sky.
The United Nations mission was not immediately available for comment.
Damascus has been rocked by several bomb blasts, including an attack last month at the national security headquarters that killed three of President Bashar al-Assad's top security chiefs.
The UN Security Council is due to meet on Thursday to discuss the future of the observer mission, whose mandate expires on August 19.
Meannwhile, Syria's former prime minister, the highest profile government figure to defect, said the regime was collapsing as Muslim countries mulled suspending Syria from the OIC.
"The Syrian regime only controls 30 percent of Syria's territory. It has collapsed militarily, economically and morally," Riad Hijab told a news conference in the Jordanian capital Amman on Tuesday.
Hijab fled to Jordan last week, the latest in a string of defections from Assad's government, which is becoming increasingly embattled as the 17-month conflict shows no signs of abating.
The United States, which has imposed a raft of tough sanctions to try to force Assad's departure, reacted by lifting an asset freeze imposed on Hijab.
"The United States encourages other officials within the Syrian government, in both the political and military ranks, to take similarly courageous steps to reject the Assad regime and stand with the Syrian people," Treasury official David Cohen said.
Hijab's replacement as premier, Wael al-Halqi, insisted the sanctions imposed by Arab and Western governments "have only affected innocent Syrians".
After Damascus talks with UN humanitarian chief Valerie Amos on what she called the "deteriorating humanitarian situation," the new prime minister voiced confidence in "Syria's capacity to resist, to overcome the crisis and to bring about reconciliation, security and stability".
In an apparent snub to Amos, he said the fate of more than a million people displaced by the conflict was "not a UN issue".
Western policymakers hope that a wave of defections will bring the collapse of the autocratic government, ending a conflict that seems to be in stalemate with the international community deeply divided over what action to take.
An emergency summit of the 57-member Organisation of Islamic Cooperation (OIC) began late Tuesday in the holy city of Mecca with a proposal to suspend Syria, a move strongly opposed by Iran.
A draft final statement obtained by AFP said the summit "approves the suspension of Syria's membership," a measure recommended by a preparatory ministerial meeting held Monday in the western Saudi city of Jeddah.
The move would further isolate Assad's regime, after Syria was suspended from the Arab League last year over its brutal crackdown on an Arab Spring-inspired revolt launched in March 2011.
The OIC represents 1.5 billion Muslims worldwide, and its suspension of Syria would heap pressure on Assad, who has characterised the uprising as a plot by Western and rival powers to overthrow his Iran-allied government.
Iranian President Mahmoud Ahmadinejad, whose country has openly criticised the push to suspend Syria, is attending the extraordinary OIC meeting and was expected to strongly oppose the move against Damascus.
The draft statement says Syria should be suspended over "the obstinacy of the Syrian authorities in following the military option" to solve the crisis and the failure of a UN-Arab League peace plan brokered by Kofi Annan.
Although a number of senior figures have abandoned the regime, analysts say that until military units begin to defect en masse, the Assad family and the top echelon of the military and security services will remain intact.
"Syria is full of officials and military leaders who are awaiting the right moment to join the revolt," Hijab said, urging the fractured opposition to unite.
His comments came as fresh fighting erupted for control of key districts of the northern city of Aleppo, a human rights watchdog said.
Aleppo -- where communications have been cut for at least three days -- is a metropolis of some 2.7 million people and seen as pivotal to the outcome of the conflict, with some referring to it as Syria's Benghazi, the Libyan city at the heart of the revolt that toppled Moamer Kadhafi's regime.
A total of 89 people were killed nationwide on Tuesday, 53 of them civilians, it said.
The opposition issued a new appeal Monday for the international community to impose no-fly zones similar to those established during the conflict in Libya amid increasing air strikes by Syrian warplanes.
The conflict has killed more than 23,000 people since March last year, according to the Observatory, while the UN says more than one million people have been displaced and another 140,000 have fled to Syria's neighbours.
Washington charged Tuesday that Tehran is working to form a pro-regime militia in Syria, similar to the Hezbollah militia it promoted in neighbouring Lebanon during that country's 1975-90 civil war.
"It is obvious that Iran has been playing a larger role in Syria in many ways," US Defence Secretary Leon Panetta told a news conference.
"There's now an indication that they're trying to develop, trying to train a militia within Syria to be able to fight on behalf of the regime," he said.