By Li Hongmei

The U.S. led a fierce denunciation on Wednesday targeting the Syrian regime and vowed it would step up support for the opposition as it seeks to topple President Bashar al-Assad.

The top US diplomat, John Kerry, set the tone when he stressed before 40 nations and international organizations 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.

At a later press conference in the Swiss city of Montreux, Kerry revealed that Washington and Moscow were also planning to work on other tracks to end the fighting which has left 130,000 people dead.

"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.

Washington has provided more than $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.

But so far it has refused to directly supply weapons and machinery to the opposition forces - now fighting against both Assad and a wave of al-Qaida extremist groups flooding into the chaos.

It remained unclear from Kerry's remarks whether the Obama administration was now prepared to review its weapons ban.

Syrian Foreign Minister Walid Muallem had earlier dubbed the country's opposition "traitors" and foreign "agents".

He hit back at Kerry's comments, saying only the Syrian people could decide their president, and offered assurances that progress had been made on allowing aid organizations access to stricken populations.