WASHINGTON (Reuters) - President Barack Obama vowed on Monday to work to bring the Syrian government and rebels to the negotiating table in coming weeks but warned that a "combustible mix" of regional meddling and Islamist militancy would make it hard to halt the country's civil war.
Even as Obama backed a new joint U.S.-Russian effort to seek a diplomatic solution in Syria, he cited an array of obstacles to a credible peace process, including the involvement of Iran, the Lebanese militant group Hezbollah and the al Qaeda-linked Nusra Front in the two-year-old conflict.
Obama, in a joint White House news conference with British Prime Minister David Cameron, injected a note of caution after Washington and Moscow raised hopes last week with an agreement to try to arrange an international peace conference on Syria.
With Syria's factional and sectarian hatreds more entrenched than ever and President Bashar al-Assad showing no sign of a willingness to give up power, it was far from clear whether the warring sides were ready to talk.
"If, in fact, we can broker a peaceful political transition that leads to Assad's departure, but a state in Syria that is still intact, that accommodates the interest of all the ethnic groups, all the religious groups inside of Syria, and that ends the bloodshed, stabilizes the situation, that's not just going to be good for us, that'll be good for everybody," Obama said.
But Obama said of the diplomatic effort: "I'm not promising that it's going to be successful."
Obama said that once "the furies have been unleashed ... it's very hard to put things back together," he said. "There are going to be enormous challenges ... even if Russia is involved, because we still have other countries like Iran, and we have nonstate actors like Hezbollah that have been actively involved."
"On the other side, we've got organizations like al-Nusra that are essentially affiliated to al Qaeda," he said. "So all that makes a combustible mix."
Obama, who has faced criticism at home and abroad for his cautious approach to the Syria crisis, made no mention of U.S. deliberations on whether to start arming Syrian opposition fighters - something he has long resisted.
"We'll continue to work to establish the facts surrounding the use of chemical weapons in Syria, and those facts will help guide our next steps," Obama said.
He has warned that use of chemical weapons by Assad's forces would be a "game-changer" but has insisted he must have conclusive proof to back intelligence assessments of probable chemical weapons deployment.
Cameron said in a an interview with National Public Radio that Britain had not ruled out taking tougher action against Assad's government, but he later told reporters that his government has not made a decision to arm the Syrian opposition.
He said, however, that Britain would double its non-lethal aid to the opposition over the next year and that it was looking at ways to provide more technical assistance to the rebels.
Obama and Cameron sought to project a united front in seeking a political solution on Syria.
"The challenges remain formidable, but we have an urgent window of opportunity before the worst fears are realized," Cameron said.
Both leaders agreed on the need to keep up pressure on Assad to step aside and make way for a political transition.
"And that includes bringing together representatives of the regime, and the opposition in Geneva in the coming weeks to agree on a transitional body, which would allow a transfer of power from Assad to this governing body," Obama said.
Secretary of State John Kerry said last week the conference could be held by the end of May, but his spokeswoman, Jen Psaki, said on Monday it could slide to early June.
Obama said he and his aides were working to narrow differences with Moscow, which has backed the Syrian government, to show Russian President Vladimir Putin it was in his country's interest to push Assad out.
"I don't think it's any secret that there remains lingering suspicions between Russia and other members of the G8 or the West," Obama said. He said he and Kerry, who met Putin in Moscow last week and secured agreement to pursue a Syria peace conference, were trying to break down those suspicions.
Additional reporting by Laura MacInnis, Susan Heavey, Roberta Rampton and Arshad Mohammed; Editing by Cynthia Osterman and Philip Barbara