Senior Syrian opposition figures are questioning whether they should attend peace talks next month following US warnings that the swift departure of President Bashar al-Assad from power cannot be guaranteed, opposition sources said on Friday.

Speaking after meeting US Ambassador Robert Ford in Istanbul this week, they said he had told them not to expect quick results from the talks, due to start in Switzerland on Jan. 22, and that it was not up to Washington to remove Assad.

The planned talks are the most serious international effort yet to find a political solution to an almost three-year-long conflict which has killed more than 100,000 people, made many millions homeless, and become a proxy war between the Middle East's Sunni and Shi'ite Muslim powers.

Assad, bolstered by Shi'te Iran and its regional allies, has recovered some of the ground he lost to Sunni Muslim rebels and his officials have ruled out surrendering any power at the Geneva 2 talks, defying opposition demands that Assad must go.

Leaders of the main political opposition in exile, the National Coalition, said they are ready to attend the talks. But they insist that the process must lead to Assad's departure, and have yet to formally agree on participation.

While reiterating the US mantra that Assad has no future in Syria, Ford warned the opposition that unless rebel brigades overcome a split between Western-backed moderate forces and Islamists who oppose the Geneva talks, Assad looked set to stay.

He also said the Geneva process was likely to go on for months, sources at the talks told Reuters.

"The message we got was that there could be no guarantees. The US said they want Assad out, but it's really got to be in the hands of the Syrian people," said a coalition member who asked not to be named because of the sensitivity of the issue.

That kind of language has given Geneva sceptics within the coalition more ammunition.

"This makes the top leadership very upset. They are sincerely evaluating whether they should be going to Geneva," he added.

Coalition Secretary General Badr Jamous said the opposition believed neither the United States nor Russia - which along with the United Nations have pushed for the Geneva talks to take place - was doing enough to ensure their success.

"They are not putting enough pressure on Assad," Jamous said.

Coalition members are due to meet on Jan. 7 to decide formally whether or not to attend the Geneva talks.

"If things don't get better coalition members may decide that we won't go," Jamous said.

Despite its grievances the coalition is in little position to resist international pressure to show up in Geneva. It has no real political power base and its military wing, the Supreme Military Command of the Western-backed Free Syrian army, has minimal influence on the ground.

But Gulf countries such as Saudi Arabia and Qatar, both prominent supporters of the political opposition and rebel fighters, are wary of Geneva and could put pressure on the coalition to stay away.

"Some of the regional powers don't want us to go," said a coalition source, normally based in Europe, referring to the two Sunni Gulf monarchies.

"These countries are not against Geneva in principle but they don't see the current climate as conducive to a successful conference," he said.

"But if they don't want us to go they should be brave enough to ask for it. We are not strong enough to ask for the process to be postponed."

Coalition member Hadi Albahra also suggested on Friday that Middle East powers might hold up next month's talks, tweeting: "Possibility to postpone the Geneva 2 Conference due to lack of regional consensus around it".

Alongside the political divisions over the Geneva talks, chronic faultlines in the rebel forces have also undermined Western efforts to pressure Assad, whose family has ruled Syria for 40 years.

Last month fighters from the Islamic Front, an alliance of six radical groups opposed to the Geneva talks, took over a main weapons depot in northern Syria which had been under the control of the Western-backed Supreme Military Council.

The United States said this week the Islamic Front had spurned its offer of direct talks and warned that unless the SMC was recognised by other rebel brigades there was little chance that they could be an effective force.

"If there is not coordination, if there is not tight co-operation between the different armed groups that are fighting the regime, the regime is going to be successful in surviving," US envoy Ford said.

That comment echoed his private message that the United States sees no role for Assad in Syria "but cannot force it", said a source who is a member of the coalitions' political committee.

A senior U.N. diplomat, speaking on condition of anonymity, echoed Ford's uncertainty about any Assad departure.

"Assad cannot be part of the transition. What is unclear is the exact timing of Assad's departure," he said. "He's going to need to disappear from the scene, at exactly what point that is is obviously what will be discussed in Geneva we hope."-Reuters