US Senator Marco Rubio urged the United States on Wednesday to provide ammunition to Syrian rebels and share intelligence, as Washington was said to weigh "non-lethal" aid.

"What the opposition really needs is access to ammunition," the prominent Republican said days after a US congressional visit to Israel, the occupied territories and Jordan.

"We can identify a couple of responsible groups... that we feel have built capacity," Rubio said at the Washington Institute for Near East Policy, a think tank in the US capital.

"Ammunition is something we can provide which is not weaponry per say, but is essential."

Amid a raging civil war pitting regime loyalists against rebels seeking to oust strongman Bashar al-Assad, Rubio warned that the best-organized and best-armed rebel groups in Syria "are the most radical ones, the most anti-democratic ones, the most anti-American ones."

"The real risk is that when Assad falls -- and he will fall -- the largest, those well-equipped, best organized groups in that conflict, will be the people that quite frankly are against our national interest."

The young but influential first-term senator, a possible 2016 presidential prospect, spoke just hours after President Barack Obama's new Secretary of State John Kerry hinted at greater US support for Syria's opposition.

On the eve of a meeting of the Friends of Syria group in Rome, Kerry said boosting support for the opposition would be a key part of the talks Thursday bringing together foreign powers and the main opposition National Coalition.

The Washington Post reported that the White House was considering a policy shift to supply rebels with "non-lethal" aid, including armored vehicles and perhaps even military training.

The United Nations says at least 70,000 people have died and hundreds of thousands have been uprooted in the two-year conflict.