Syrian forces struck back against rebels today with attack helicopters and shelling in Damascus, one day after an audacious rebel attack in the capital killed three leaders of the regime.

The whereabouts of President Bashar Assad, his wife and their three young children were not known.

Although Mr Assad does not appear in public frequently, his absence was notable following such a serious blow his inner circle.

Thousands of Syrians streamed across the Syrian border into Lebanon, fleeing as fighting in the capital entered its fifth straight day, witnesses said. Residents near the Masnaa crossing point - about 25 miles (40 kilometres) from Damascus - said hundreds of private cars as well as taxis and buses were ferrying people across.

Maj Gen Robert Mood, the Norwegian head of nearly 300 unarmed UN observers in Syria, condemned the violence and encouraged a diplomatic solution, which appears increasingly out of reach.

He spoke just hours before a planned UN Security council vote on whether to renew the mission's mandate, which expires tomorrow, and impose new sanctions on the Damascus regime.

"It pains me to say, but we are not on the track for peace in Syria," Gen Mood said in Damascus.

The UN vote had been scheduled for yesterday, but was postponed after key Western nations and Russia failed to agree the text of a resolution aimed ending the escalating violence. Russia, a long-time ally of Syria, has stood by the Syrian regime and vowed to veto any measures that could lead to international military intervention.

In today's fighting in Damascus, government forces fired heavy machine guns and mortars in battles with rebels in a number of neighbourhoods in the capital, the Britain-based Syrian Observatory for Human Rights said.

Adding to the confusion, Syria's state-run TV warned citizens that gunmen were disguising themselves in military uniforms to carry out attacks.

"Gunmen are wearing Republican Guard uniforms in the neighbourhoods of Tadamon, Midan, Qaa and Nahr Aisha, proving that they are planning attacks and crimes," SANA said.

Many residents were fleeing Damascus' Mezzeh neighbourhood 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 unarmed observers were authorised for 90 days to monitor a ceasefire and implementation of UN envoy Kofi Annan's six-point peace plan, but the truce never took hold and the monitors have found themselves largely locked down because of the persistent violence.

Gen 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 rebels fighting to topple Mr Assad.

Yesterday's rebel bomb attack on high-level crisis meeting struck the harshest blow yet at the heart of the regime. The White House said the bombing showed Mr Assad was "losing control" of Syria.

Syrian TV confirmed the deaths of Defence 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 defence minister who is married to Mr Assad's elder sister, Bushra, and is one of the most feared figures in the inner circle; and Hassan Turkmani, 77, a former defence 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.

Mr Assad has attended the swearing-in of a new defence minister, the state news agency said.

It did not specify exactly where the swearing-in took place. But it is the first word on Mr Assad since the bombing.

State-run TV says Mr Assad gave the new defence minister "instructions" and wished him luck.