* U.S. sees focus back on "Friends of Syria"
* Chemical weapon threat tops U.S. security concerns
* Republicans slam Obama for lack of leadership
By Andrew Quinn
WASHINGTON, July 19 (Reuters) - Beaten back by a double veto at the United Nations, the United States on Thursday sought to rescue its Syria strategy amid fears that the crisis was lurching into a dan gerous en dgame.
The Obama administration had fought for a U.N. Security Council resolution setting out consequences for Syrian President Bashar al-Assad's failure to halt his crackdown, saying this was essential if international mediator Kofi Annan's fragile political transition plan was ever to take hold.
But Russia and China, as they have twice before, blocked the proposed resolution -- leaving Washington and its allies facing a dwindling array of options amid scant appetite for direct military intervention.
"Clearly they are stymied," said Brian Katulis, a security expert at the Center for American Progress, a Washington think tank close to the Obama administration.
"While they will do what they can for the opposition, they are now likely to focus on core U.S. security interests such as chemical weapons, the al Qaeda element and the danger that this could spill over into a wider conflict."
Fears of a Syrian meltdown have intensified following Wednesday's bomb attack which killed three of Assad's top aides, and rebel forces have battled deep into the heart of Damascus even as the government unleashed artillery and helicopter gunships in an attempt to secure the capital..
Russia and China have opposed tough U.N. measures, seeing them as part of a U.S.-led effort to stage-manage the fall of Assad, long one of Moscow's chief regional allies.
FRIENDS IN FOCUS
With the violence increasing and impasse at the United Nations, U.S. officials said the diplomatic focus would now shift back to alternatives such as the "Friends of Syria," the coalition of western countries, Arab states and key neighbor Turkey which has sought to squeeze the Assad government through sanctions and build up the country's disor ganized oppo sition.
"We will intensify our work with a diverse range of partners outside the Security Council to bring pressure to bear on the Assad regime," U.S. Ambassador to the United Nations Susan Rice said following the vetoes on Thursday.
Some members of the group, including Saudi Arabia and Qatar, are believed to be aiding the flow of weapons to Syria's rebel forces. But U.S. officials say Washington will likely steer clear of such efforts given the murky nature of the rebellion and the uncertainties of a wider military conflict.
"We prefer a managed political transition that really is the only route to stop the violence, end the bloodshed and get us to a new Syria and a better day. But we're preparing for all scenarios," State Department spokesman Patrick Ventrell said.
Among the scenarios that most alarm U.S. officials is the potential that Syria's government will either lose control of its stockpile of chemical weapons -- believed to be the largest in the Middle East -- or put them to use on the battlefield in a last-ditch effort to crush the rebellion.
The White House, the Pentagon and the State Department have all warned Damascus that it must secure the chemical weapons stockpiles, and U.S. officials said Washington was conferring with Syria's neighbors including Israel on what could emerge as a broader regional threat.
"We're actively consulting Syria's neighbors and our friends in the international community to underscore our common concern about the security of these weapons," the State Department's Ventrell said.
Other concerns include reports that al Qaeda fighters are working alongside Syria's rebels, and that the escalating humanitarian crisis and refugee flows could destabilize fragile neighbors such as Lebanon and Jordan.
CALLS FOR LEADERSHIP
With the United States bearing down on its November presidential election, President Barack Obama's domestic critics have also stepped up their calls for a more assertive U.S. stance -- although Republicans themselves remain divided on what the next steps should be.
Obama's Republican challenger, Mitt Romney, on Thursday said Obama had abdicated U.S. leadership.
"While Russia and Iran have rushed to support Bashar al-Assad and thousands have been slaughtered, President Obama has abdicated leadership and subcontracted U.S. policy to Kofi Annan and the United Nations," Romney said in a statement.
Republican Senator John McCain, who has urged air strikes on Assad's forces, said Washington risked losing its influence with Syria's opposition if it stays on the sidelines.
But House of Representatives Republican leader John Boehner said the White House was right to remain cautious -- reflecting widespread fatigue with overseas military involvement following the conflicts in Iraq and Afghanistan.
"I believe that Assad has to go but I don't think that we need to overly involve ourselves to the extent of direct military action," Boehner told CNN.
Some analysts said, however, that the United States and its allies might yet be drawn more directly into the Syrian conflict, with or without U.N. approval.
"The Friends of Syria could become the coalition of the willing. You lay down sanctions, and then you come up with military contingencies, that is Plan B," said Andrew Tabler, a Syria expert at the Washington Institute for Near East Policy.
"It is going to be driven by necessity, and by what is going on on the ground," Tabler said. "If the threats are genocide, and chemical weapons are used, that is going to change things."
(Additional reporting by Donna Smith, Rick Cowan and Steve Holland; Editing by Mary Milliken and Cynthia Osterman)