The main Syrian rebel group has claimed responsibilty for an explosions in central Damascus near the hotel used by the UN observer mission in Syria.
Abu al-Noor, a spokesperson for the Ahfad Al Rasoul Brigade that has claiming responsibility for the bombing, told Al Jazeera that the Free Syrian Army (FSA) had been planning the strike for a month.
"The operation was targeting the central security command in response to murders perpetrated by the security forces nationwide," he said in a phone interview from Damascus on Wednesday morning.
Syrian state television reported that the blast, in the Abu Remanih area near a military depot and the trade union headquarters, was caused by a bomb planted in a fuel vehicle on Wednesday morning.
Several ambulances rushed to the scene and a security cordon was set up, according to witnesses.
Syrian state television reported that the attack wounded at least three people, but the rebels said they suspect "many" officers had been killed or injured.
The FSA planted eight explosive canisters in the vicinity of the Syrian military's Central Security Command, timed to explode during their daily meeting, said Abu al-Noor of the Ahfad Al Rasoul Brigade.
The FSA had intelligence suggesting some 150 high-ranking officers would be attending the meeting. He did not know how many casualties had occurred because the security forces had secured the area.
"We will continue to carry out similar operations in the capital until we reach him [Assad] in the presidential palace," Abu al-Noor said.
Several hours after the blast, anti-government activists said that rebels clashed with Syrian forces near the government headquarters and the Iranian embassy in the Mezze area of Damascus.
Faisal Mekdad, the Syrian deputy minister, toured the area of the blast shortly afterwareds and said none of the UN staff was hurt.
"This is a criminal act that shows what kind of attacks Syria is being subjected to," he said at the scene. "Such explosions will not affect Syria."
"I confirm that we are with the UN and we will do all we can to guarantee their protection so that they carry out their role."
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General Babacar Gaye, the head of the UN Supervision Mission in Syria (UNSMIS), had held a press conference in the hotel, the Dama Rose, on Monday.
UNSMIS was not immediately available for comment.
The explosion occurred as Valerie Amos, the UN humanitarian chief, was in the city, but her team is believed to be staying at a different hotel.
Al Jazeera's Rula Amin, reporting from Beirut in neighbouring Lebanon, said the FSA's apparent incursion into the heart of Damascus showed that it was gaining inconfidence.
"If indeed this is the work of the Free Syrian Army, this is a breakthrough for them," she said. "What's interesting is that this is a very secure area."
Plume of smoke
Opposition activists said massive explosions echoed across much of the city and a video released by one opposition group showed a large plume of smoke billowing into the sky.
A spokesperson for the opposition Local Co-ordination Committees in Damascus told Al Jazeera there was a big explosion followed by a series of smaller explosions.
Shareef Shihada, a member of Syria's parliament, said only few people injured and some damage. He said the explosion had not hit a military target.
Damascus has been rocked by several bomb blasts in recent days, including an attack last month at the national security headquarters that killed three of President Bashar al-Assad's security commanders.
The UN Security Council is due to meet on Thursday to discuss the future of UNSMIS, whose mandate expires on August 19.
On the diplomatic front, the US defence secretary, Leon Panetta, accused Iran on Tuesday of setting up pro-government militias in Syria.
Iran has made similar claims of intervention in the conflict by the US and its allies.