By David Brunnstrom
BRUSSELS, Dec 5 (Reuters) - The United States is worried that an increasingly desperate Syrian President Bashar al-Assad could resort to the use of chemical weapons against rebels, or lose control of them, U.S. Secretary of State Hillary Clinton said on Wednesday.
After a meeting of NATO foreign ministers at which the Western military alliance agreed to send Patriot anti-missile batteries to Syria's neighbour, Turkey, Clinton said Washington had made clear to Syria that use of chemical arms would be a "red line" for the United States.
"Our concerns are that an increasingly desperate Assad regime might turn to chemical weapons, or might lose control of them to one of the many groups that are now operating within Syria," Clinton told a news conference.
"And so as part of the absolute unity that we all have on this issue we have sent an unmistakable message that this would cross a red line and those responsible would be held to account."
Saying a political transition in Syria needed to start as soon as possible, Clinton said the United States would do what it could to support Assad's adversaries, now that a new opposition coalition has been formed.
The United States and other countries will discuss at a meeting of the Friends of Syria group in Marrakesh, Morocco next week what more they could do to try to bring the Syrian conflict to an end, she said.
"But that will require the Assad regime making the decision to participate in a political transition (and) ending the violence against its own people ... We hope that they do so because we believe ... that their fall is inevitable. It is just a question of how many people will die until that date occurs."
The National Coalition for Syrian Opposition and Revolutionary Forces was set up earlier this month in an attempt to unify Assad's fractured opponents and win greater international support, and possibly arms.
While the coalition has won formal recognition from Turkey, France, Britain and Gulf Arab states, the United States has so far stopped short of this step.
Other nations will be watching carefully to see whether Washington will throw its full support behind the coalition by recognising it as "the" representative of the Syrian people, a step that might open the way for greater aid flows.
Clinton said that with the new coalition, the United States was "going to be doing what we can to support that opposition".
The West has been wary in dealing with the Syrian opposition in exile due to concerns that some groups lack support on the ground in Syria and worries about Islamic radicals in the rebel ranks.