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 favor 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 UN 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.
UN investigators have been gathering evidence of atrocities committed by rebels as well as by Assad loyalists.
Brahimi, the UN-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."