BEIRUT — Syrian government forces struck back against rebels with attack helicopters and shelling around Damascus Thursday after an audacious bomb attack that killed three senior members of the ruling regime.

The whereabouts of President Bashar Assad, who has not been seen publically since Wednesday’s blast, his wife and his three young children remained unknown.

As fighting raged in the capital for a fifth day, the chief U.N. observer warned Thursday that Syria was not “on the track for peace.”

The Britain-based Syrian Observatory for Human Rights said government forces were firing heavy machine guns and mortar shells and fighting with rebels in a number of neighborhoods in the capital.

Many residents were fleeing the Mezzeh neighborhood after troops surrounded it and posted snipers on rooftops while exchanging gunfire with opposition forces.

The Observatory, which relies on a network of activists inside Syria, said rebels damaged one helicopter and disabled three military vehicles.

Rebels fired rocket-propelled grenades at a police station in the Jdeidet Artouz area, killing at least five officers, the group said.

Activist claims could not be independently verified. The Syrian government bars most media from working in the country.

Maj. Gen. Robert Mood, the Norwegian head of nearly 300 observers sent to the country to monitor a cease-fire that never took effect, said the mission was not working. His comments came ahead of a planned U.N. Security council vote on whether to renew the mission’s mandate, which expires Friday, and impose new sanctions on the Damascus regime.

That vote had been scheduled for Wednesday but was postponed after key Western nations and Russia failed to agree the text of a resolution aimed ending the escalating violence.

“It pains me to say, but we are not on the track for peace in Syria,” Mood told reporters in Damascus.

The unarmed observers were authorized for 90 days to monitor a cease-fire and implementation of Kofi Annan’s six-point peace plan, but the truce never took hold and they have found themselves largely locked down because of the persistent violence.

Mood said the observers “will become relevant when the political process takes off.”

Syria’s 16-month crisis began with protests inspired by the Arab Spring wave of revolutions, but it has evolved into a civil war, with scores of rebel groups fighting to topple Assad.

Wednesday’s rebel bomb attack on high-level crisis meeting struck the harshest blow yet at the heart of Assad’s regime. The White House said the bombing showed Assad was “losing control” of Syria.

Syrian TV confirmed the deaths of Defense Minister Dawoud Rajha, 65, a former army general and the most senior government official to be killed in the rebels’ battle to oust Assad; Gen. Assef Shawkat, 62, the deputy defense minister who is married to Assad’s elder sister, Bushra, and is one of the most feared figures in the inner circle; and Hassan Turkmani, 77, a former defense minister who died of his wounds in the hospital.

Also wounded were Interior Minister Mohammed Shaar and Maj. Gen. Hisham Ikhtiar, who heads the National Security Department. State TV said both were in stable condition.

Rebels claimed responsibility, saying they targeted the room where the top government security officials in charge of crushing the revolt were meeting.

Activists say more than 17,000 people have been killed, most of them civilians. The Syrian government says more than 4,000 security officers have been killed. It does not given numbers of civilian dead.