* Cameron suggests Assad exit deal "could be arranged"
* Brahimi warns Syria could become a new Somalia
* Renewed violence kills more than 100
Britain floated the notion on Tuesday of Syria's President Bashar al-Assad leaving power with immunity from prosecution while the opposition said at least 100 more people were killed in the country's 19-month revolt.
"Anything, anything, to get that man out of the country and to have a safe transition in Syria," British Prime Minister David Cameron told Al Arabiya news network in Abu Dhabi before flying to Saudi Arabia.
Peace envoy Lakhdar Brahimi warned that Syria, where some 32,000 people have died in the revolt against Assad, could end up a collapsed state like Somalia, prey to warlords and militias.
More than 100 people were killed across the country on Tuesday, the Syrian Observatory for Human Rights said.
Among them was the brother of Syria's parliament speaker, assassinated by gunmen in Damascus, state television said. He was the latest victim in a deadly campaign against perceived Assad supporters and their families.
Air strikes killed 17 people, including women and children, in the Damascus suburb of Kfar Batna, according to the Syrian Observatory, an opposition watchdog based in Britain.
Video footage of the raid's aftermath, which could not be verified, showed a toddler with a severed head and the torso of a young man, his head and limbs gathered near him by rescuers.
Insurgents killed 12 soldiers and wounded 20 in an attack on a convoy of off-road vehicles in the northern province of Idlib.
Air strikes and artillery barrages unleashed by the Syrian military in the last few weeks have devastated whole districts of the capital, as well as parts of towns and cities elsewhere.
Yet, for all their firepower, Assad's forces seem no closer to crushing their lightly armed opponents, who in turn have so far proved unable to topple the Syrian leader.
"Of course I would favour him facing the full force of international law and justice for what he's done," Cameron said of Assad. "I am certainly not offering him an exit plan to Britain but if he wants to leave he could leave, that could be arranged."
It was unclear if Cameron had spoken to other U.N. Security Council members about the idea - which could involve offering Assad immunity from prosecution if he accepted asylum in a third country. Nor was it clear what nation would take him.
The U.N. human rights office has said Syrian officials suspected of committing or ordering crimes against humanity should face prosecution at the International Criminal Court.
U.N. investigators have been gathering evidence of atrocities committed by rebels as well as by Assad loyalists.
Brahimi, the U.N.-Arab League envoy to Syria, told the London-based al-Hayat newspaper he did not expect ethnic or sectarian partition there. "What I am afraid of is worse ... the collapse of the state and that Syria turns into a new Somalia."
At the United Nations, diplomats cited a senior U.N. official as telling the Security Council that Brahimi had urged Russia to be "more pro-active" in resolving the Syrian crisis.
In a closed-door session of the 15-nation council, U.N. political affairs chief Jeffrey Feltman also said he had received credible reports of the use of cluster bombs by Syrian government forces, the envoys reported.
Big powers and regional nations are split over Syria. Russia and China have blocked three Western-backed U.N. Security Council draft resolutions aimed at exerting pressure on Assad.
Brahimi, speaking in Cairo on Sunday, called on the council to adopt a resolution based on an understanding brokered by his predecessor Kofi Annan in Geneva in June which called for the establishment of a transitional government in Syria.
The Geneva Declaration did not specify what role, if any, Assad would play in a future Syria.
Russian Foreign Minister Sergei Lavrov urged the Syrian opposition to enter talks with the authorities to end the crisis and abandon a precondition that Assad step down.
"The most important thing is stopping the violence immediately. If it is more important to the other side to change the Assad regime, then they want to continue the bloodbath in Syria," Lavrov said in Amman after meeting former Syrian Prime Minister Riad Hijab, who defected to Jordan in August.
Hijab said Assad's removal was "the only way out".
Assad's foes have also failed to unite, making it harder for the outside world to support or arm them.
Opposition factions were meeting in Qatar in an effort to forge a common front between civilians and rebels, Islamists and secularists, as well as groups outside and inside Syria.
Prominent dissident Riad Seif has proposed a new 50-member unity council, but the head of the widely criticised Syrian National Council (SNC), which is based abroad, said it should retain a "central role" in any opposition configuration.
A Doha-based diplomat said SNC members feared their group risked losing influence in the new civilian body, which would later choose an interim government and coordinate with armed rebel groups. Seif's initiative is to be debated on Thursday.
Syria has accused Qatar, Saudi Arabia and Turkey, among others, of fuelling the bloodshed by backing the rebels.
The Syrian struggle has taken on a sectarian tone, with mostly Sunni rebels battling loyalist forces dominated by Assad's minority Alawite sect, an offshoot of Shi'ite Islam.
Saudi Arabia and other Sunni Gulf Arab states are wary of powerful Shi'ite neighbour Iran, one of Assad's few allies.
Syria's neighbours all fear spillover from the conflict in the shape of refugee inflows, cross-border violence or a wider upheaval igniting Sunni-Shi'ite tensions across the region.
In Turkey, fiercely critical of Assad, the state-run news agency reported the arrival overnight of seven Syrian army generals who had defected and crossed the border.
Scores of Syrian officers have defected to Turkey since the uprising began. About 112,000 Syrian refugees are housed in camps along Turkey's 900-km (560-mile) border with Syria.