The Syrian opposition gave a largely negative response on Sunday to the transition deal hammered out by world powers aimed at resolving the bloody conflict, with a former top leader branding it a "farce."

The ex-head of the umbrella Syrian National Council, Burhan Ghalioun, described as a "mockery" the notion that Syrians should negotiate with "their executioner, who has not stopped killing, torturing... and raping women for 16 months."

"The Syrian people only have one option now, and that is to fight a war of popular liberation," he said, describing the Geneva deal as a "farce," according to the opposition coalition's official Facebook page.

Ghalioun, who resigned in May but remains a senior SNC executive board member, told pan-Arab television Al-Arabiya that "this is the worst international statement yet to emerge from talks on Syria."

SNC spokeswoman Basma Qadmani was more cautious, however, saying the plan drawn up by international peace envoy Kofi Annan and agreed at the multi-nation meeting in Geneva on Saturday had "a few positive elements."

But she told AFP in Ankara: "Important elements remain too ambiguous... and the plan is too vague to foresee real and immediate action."

The Geneva meeting agreed on a plan for a transition in Syria that could include current regime members, but the West ruled out any role for President Bashar-al Assad in a new unity government.

And Russia and China, which have twice blocked UN Security Council resolutions on Syria, insisted that Syrians themselves must decide how the transition is carried out rather than allow others to dictate their fate.

However, both Moscow and Beijing signed up to the deal, drawn up after 16 months of violence in Syria that activists say has killed more than 15,800 people.

"There are two positive elements," Qadmani said in a telephone interview.

"The first one is that the final declaration says that the participants agree to say that the Assad family cannot rule the country any more, and therefore the Assad family cannot lead the transition period."

"The second positive element is the agreement that the transition should comply with the legitimate aspirations of Syrian people.

"For us this means that Assad should go because Syrian people have already said that they want Assad to go."

But she added: "We are totally opposed to the fact that stopping violence is not a precondition to the political process."

The Local Coordination Committees, which organise protests on the ground in Syria, said the outcome showed once again the international community's failure to adopt a common position.

It called the transition accord "just one version, different in form only, of the demands of Russian leaders allied to the Assad regime and who cover it militarily and politically in the face of international pressure."

"The new agreement contains obscure turns of phrase that give the Assad regime's gangs another chance to play for time in suppressing the popular revolution and to silence it through violence and massacres."

Another SNC member, Khaled Khoja, was also critical of the Geneva deal.

"As SNC, we refuse any initiative that does not say clearly Bashar al-Assad and his killer team leave power," he told AFP in English, saying that was a precondition before any dialogue.