The United States should look at providing weapons and other aid to Syrian rebels if Russia and China refuse to reconsider their vetoes of a UN resolution against Damascus, a top US senator said Sunday.

Senator Joe Lieberman, a former Democratic presidential candidate, said the Syrian people "have shown extraordinary courage in the face of a government much stronger than they are" and "they are not going to be denied."

"The question is not whether (President Bashar Al-Assad) will go, but when he will go. The sooner he goes, the fewer innocents will be killed," he said at security conference in the southern German city of Munich.

Russia and China faced worldwide outrage after they blocked a UN Security Council resolution on Saturday condemning Syria for its crackdown on protests amid a new bout of violence.

"If Russia and China don't change their minds about the veto ... then the world will not allow us to say there's nothing we can do about it," Lieberman said.

"So we should begin thinking about what we can do, particularly with the Arab League," he said. "I think it begins with support for the Syrian Free Army."

The hawkish senator said a "range of support" could be given to the rebels, from medical supplies to intelligence and reconnaissance surveillance.

"And then ultimately it is providing them with weapons," he said during a panel discussion on the Middle East.

US Secretary of State Hillary Clinton said Saturday that military action in Syria "has been absolutely ruled out."

Turkish Foreign Minister Ahmet Davutoglu, speaking on the same panel as Lieberman, denied that his country was preparing military contingency plans to deal with the crisis in neighbouring Syria.

But he promised that Syrians fleeing the violence will always "have safe haven in Turkey."