WASHINGTON — A new blueprint for political transition in Syria offers the opposition "ironclad guarantees" that President Bashar al-Assad will not be among the nation's new leaders, a US official said Monday.

Syrian opposition leaders were meeting in Cairo meanwhile to chart a common vision for their country torn apart by some 16 months of fighting as they battle to oust long-time leader Assad, who so far has refused to step down.

They have been heavily critical, however, of the new compromise plan drawn up by UN-Arab League special envoy Kofi Annan and agreed in Geneva over the weekend that did not make an explicit call for Assad to step down.

The opposition Syrian National Council said on Sunday that "no initiative can receive the Syrian people's backing unless it specifically demands the fall of Bashar al-Assad and his clique."

But US State Department spokeswoman Victoria Nuland said the plan contained a clause that members of a transitional governing body to run Syria had to be agreed "by mutual consent."

"So from our perspective... there is no way that Assad, his cronies or anybody with blood on their hands is going to meet the mutual consent standard," she told reporters.

"This express, written assurance gives them ironclad guarantees that they will be able to veto people like Assad."

She also said the plan should appeal to members of the current government in a bid to get them to break with "the bloody tactics of the regime and join this kind of an institution, to step forward and start working on it."

It should "appeal to those people serving in the government, serving in the military, who have grave discomfort with what Assad has been up to but worry... about preserving the institutions of the state, worry about being able to have a say in the future of their country," Nuland added.

Annan was now reaching out to his contacts within Syria to try to identify members of a transitional body to govern the country.

"The mandate that the group gave to Kofi Annan was now to roll up his sleeves and begin working with all sides to come up with candidates, to try to come up with a body that can actually move Syria beyond Assad," Nuland added.