By Alistair Lyon
LONDON (Reuters) - If anyone saw last week's U.S.-Russian agreement to convene a peace conference on Syria as a potential breakthrough, Western leaders have been going out of their way to disabuse them.
International envoy Lakhdar Brahimi hailed the plan as the "first hopeful news" on Syria in a long time and deferred his own plans to resign after nine months of futile mediation.
He called the proposal "only a first step". But even its sponsors are dampening expectations that a civil war estimated to have killed 80,000 people can be doused soon, and pitfalls they cite in public are only a few of those lying in wait.
"I'm not promising that it's going to be successful," U.S. President Barack Obama said on Monday. Obstacles include Iran and Lebanon's Hezbollah, both of which support President Bashar al-Assad, as well as the al Qaeda-linked al-Nusra Front on the rebel side, he added.
Obama did not mention chronic disunity in the ranks of the Western-backed opposition or its almost complete lack of control over the now mostly Islamist insurgent forces on the ground.
Once "the furies have been unleashed ... it's very hard to put things back together", he said.
Syria has descended into a ferocious civil war, whose sectarian dimension was illustrated at the weekend by a video showing a Sunni Muslim rebel commander cutting out and biting into the heart of a slain Alawite soldier.
The United States and Russia share interests in Middle Eastern stability and in curbing Islamist militancy, but remain far apart on how to pacify Syria and shape its political future.
On Tuesday, Russian President Vladimir Putin, who opposes foreign military intervention or arming the rebels, said: "It is extremely important to avoid any actions that could aggravate the situation."
The conflict is complicated and fuelled by regional conflicts, some with sectarian overtones, such as the struggle between Sunni heavyweight Saudi Arabia and Shi'ite Iran.
Moscow, which has shielded Assad diplomatically since mostly secular peaceful protests against him erupted in March 2011, has long echoed the Syrian leader's line that what later turned into an armed revolt is the work of foreign-backed Islamists.
Russia says Assad's survival in power is not its goal, but insists his removal must not be a precondition for talks.
A Russian official said at the weekend there was broad agreement that the Syrian crisis was dire, "beyond that there are very many differences: who can take part in this format, who is legitimate and who is not legitimate".
French Foreign Minister Laurent Fabius questioned whether the Geneva talks aimed at creating a transitional government that would take over Assad's powers would even happen.
"I'm supporting the 'Geneva 2' talks, but it's extremely difficult," Fabius told RTL radio on Tuesday.
Tentative cooperation between Washington and Moscow might help - Brahimi's predecessor Kofi Annan quit last year in frustration at the diplomatic paralysis caused by big power divisions - but even acting in concert they might be impotent to staunch a conflict already spilling over to Syria's neighbors.
It remains to be seen if they can cajole their deeply skeptical Syrian allies into joining the Geneva negotiations, whose earliest timing has now slipped from May to early June.
The main opposition coalition, backed by Western and some Arab states, meets in Istanbul on May 23 to decide its stance. Previously it has demanded Assad's exit before any talks, but Washington now seems ready to leave his future to negotiations.
A French official, who asked not to be named, said rivalry between Qatar and Saudi Arabia, the two main Arab sponsors of Assad's enemies, was hampering the emergence of a credible new opposition leader with a mandate to negotiate.
"It's vital that they get someone that could be at the table," the official said. "They know that continuing disunity among the opposition doesn't work. It's not just about Assad, the Free Syrian Army and the Islamists - the Syrian people need to be represented politically by the opposition."
Jordan said on Tuesday that it would host a meeting next week of the rebels' allies in the "Friends of Syria".
Assad himself, buoyed by military gains against rebel strongholds in recent weeks, seems determined to cling to power.
Information Minister Amran Zoabi said Assad's leadership role was a decision "only for the Syrian people and the ballot box". He said Syria wants specifics on the Geneva talks before deciding whether to attend.
U.S. Secretary of State John Kerry said staying away would be "another one of President Assad's gross miscalculations", but added: "I don't believe that that is the case at this moment. The Russians, Foreign Minister Sergei Lavrov has already given him the names of people who will negotiate."
Western powers want to step up pressure on Assad to hasten his fall, but have no appetite for the huge risks and costs of direct military intervention and have stopped short of arming fractured rebel factions, who now seem on the back foot.
Although France and Britain want the European Union to ease its weapons embargo on Syria to allow some arms supplies to rebels, it is hard to imagine how this would swiftly swing the military balance against Assad, whose forces are bolstered by Russian hardware and help from Iran and Hezbollah.
Nor is it clear that arms sent to those rebels Westerners see as moderates could dent the influence of the Islamist militants now spearheading the struggle - and spreading alarm in neighboring states Jordan, Iraq, Turkey, Lebanon and Israel.
The devastating conflict in Syria, now well into its third year, may have prompted a new international initiative. But the talks would only be a start, as the French official made plain, saying: "Let's be clear, even if we do have this conference, it doesn't mean there will be peace in Syria."
(Additional reporting by Patricia Zengerle in Stockholm, Oliver Holmes in Beirut, John Irish in Paris, Matt Spetalnick in Washington, Steve Gutterman in Moscow; Writing by Alistair Lyon; Editing by Alastair Macdonald)