Opposition fighters clashed with regime forces on Thursday in the Syrian capital of Damascus for the fifth straight day.

Activists reported the continued use of tanks and helicopters in neighborhoods in and around the capital, including one close to the presidential palace.

The continued fighting followed a stunning bomb attack on Wednesday on President Bashar al-Assad's inner circle that left at least three top aides dead.

The violence has been the worst seen in Damascus since the uprising began 17 months ago.

The London-based Syrian Observatory for Human Rights reported today that residents were fleeing the upscale neighborhood of Mezzeh near the presidential palace amid clashes, and helicopters were hovering over numerous parts of the city.

"It's very dangerous," Fares Mohamad, a spokesman for the opposition Local Coordination Committees, told ABC News. "They're shelling with tanks, helicopters [and] a lot of heavy weapons."

Assad has not been seen or heard from since an explosion went off in the national security building on Wednesday, killing at least three top officials. State media reported the deaths of Defense Minister Dawood Rajiah, Deputy Defense Minister Asef Shawkat and Assistant Vice President Hassan Turkmani.

The death of Shawkat, Assad's brother-in-law and one of his closest confidantes, was a particularly painful blow for the regime. It highlighted the opposition's ability to infiltrate Assad's inner circle and perhaps get to the president himself.

Opposition activists told the Reuters news agency that Assad was in his palace in the coastal city of Latakia, while aides said was working in Damascus. Syria's ambassador denied reports that Assad's wife, Asma, had fled to ally Russia.

The opposition Syrian National Council denied initial reports the attack was carried out by a suicide bomber. SNC member Ausama Monajed told ABC News the explosives were planted and remotely detonated, a plan that had been in works for two months. But 24 hours after the brazen attack, details were still unclear.

The army said Thursday that it is committed to "decisively eliminating the criminal and murder gangs and chasing them out of their rotten hideouts, wherever they are, until clearing the homeland of their evils."

State television also warned that rebel fighters were wearing military uniforms, which "confirms they are planning to commit crimes and attack people, exploiting the trust of citizens in our courageous armed forces."

The United Nations Security Council re-scheduled a vote for Thursday on a new resolution to end the violence on Syria. But deep divisions still remain between the Western members of the council and Russia - backed by China - which has blocked moves aimed at regime change. The deadlock has threatened the U.N.'s two month-long observer mission in Syria, recently suspended because of an uptick in the violence.

"We are not on the track for peace in Syria, and the escalations we have witnessed in Damascus over the past few days is a testimony to that," said the head of the mission, Gen. Robert Mood, in a farewell statement. "I leave satisfied that I, and some 400 brave women and men, have done our best under very challenging circumstances."

In the wake of the bombing, opponents of Assad on Wednesday piled on, saying the bomb attack was proof of Assad losing control.

"It is precisely because of the ongoing campaign by President Assad against his own people that we are seeing a situation that is getting worse and worse," White House spokesman Jay Carney told reporters. "That is why it is so important for the international community to come together around a plan that produces the transition - the political transition - that is essential if Syria is to have a brighter future."