Syria's main opposition negotiating group would reject any deal struck by Russia and the United States on Syria's fate that was very different from its own proposed transition plan, the group's general coordinator Riyad Hijab said on Wednesday.
The High Negotiations Committee, which is backed by Saudi Arabia and Western powers and has been involved in stalled UN-mediated peace talks, was presenting its road map to a new political settlement for Syria in London.
The proposed process would start with six months of negotiations to set up a transitional administration made up of figures from the opposition, the government, and civil society. It would require President Bashar al-Assad to leave office at the end of those six months.
The transitional body would then run the country for 18 months, after which there would be elections.
"If what the Russians and the Americans agree upon is very much different from what the Syrians aspire to, then we shall not accept it," Hijab said.
"It's not a question of keeping Assad in for six months or one month or one day, in this transitional period. The Russians and Americans know that. They know the position of the Syrian people, they have sacrificed a lot and they will not give up this demand."
Britain's Foreign Secretary Boris Johnson said the group's proposals offered the first credible picture of a peaceful Syria without Assad.
"There is still a chance that this vision can be made to work," Johnson wrote in a column in Wednesday's Times newspaper.
"If the Russians and Americans can together create a ceasefire, then the talks can restart in Geneva with the difference, perhaps, that all sides will by then have seen at least the scaffolding of a post-Assad Syria."
Moscow and Washington are backing opposite sides in the 5-1/2-year-old Syrian conflict, with the Russians fighting on Assad's side while the Americans back opposition groups and insist Assad must go.
The two powers have been negotiating in recent days, with Presidents Barack Obama and Vladimir Putin meeting for 90 minutes on Monday on the sidelines of a G20 meeting in China, but failed to reach an agreement.
Hijab said the greatest challenge in achieving political transition came from the international community, pointing the finger at Iran's Revolutionary Guards, other regional militias, and above all, Russia for protecting Assad.
"What Mr. Lavrov has put on the table about Assad will not solve the problem," Hijab said, accusing Moscow of using proscribed weapons to shore up Assad's regime.
Regional power Iran, arch-enemy of Saudi Arabia, is Assad's strongest supporter and is determined to keep him in power in Damascus.
US Defense Secretary Ash Carter said US diplomats were trying to persuade Russia to take steps toward a true ceasefire in Syria and to nudge Damascus toward a political transition, but news out of Syria was "not encouraging."
Efforts are still going on behind the scenes, and Saudi Arabia's Foreign Minister Adel al-Jubeir said in London on Tuesday that an agreement was possible within 24 hours.
In a briefing with British reporters, Al-Jubeir said it was right to "leave no stone unturned" in the quest for a political solution in Syria, but he appeared pessimistic. He repeatedly said Assad was not trustworthy and was unlikely to comply with any agreement.
Writing by Estelle Shirbon; editing by Ralph Boulton.