The top US diplomat, John Kerry, set the tone when he stressed before 40 nations and international organisations gathered at a landmark peace conference in Switzerland that Assad could play no part in Syria's future leadership.
"There is no way -- no way possible in the imagination -- that the man who has led the brutal response to his own people could regain the legitimacy to govern," Kerry insisted.
"One man and those who have supported him can no longer hold an entire nation and a region hostage," said the US secretary of state, who has led efforts with his Russian counterpart Sergei Lavrov to launch the peace talks and end the three-year war.
"There will be parallel efforts being made, even while the talks are going on, to find different pressure points and find a solution," he told reporters from the world's media, refusing to go into detail.
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"I will just say to you that lots of different avenues will be pursued, including continued support, augmented support to the opposition."
Washington has provided more than USD 1.3 billion in humanitarian assistance to the Syrian people, and has been supplying non-lethal materiel such as body armour, communications equipment and night-vision googles to the armed rebels.