Few Americans want to see their country dragged into another war of complicated loyalties and sectarian rivalries in the Muslim world, a little more than a year after leaving Iraq and with 66,000 U.S. combat troops still in Afghanistan.
Administration officials say they don't have enough assurances that rebel units under the sway of Islamic fundamentalists won't turn their weapons on Israel or other U.S. allies and fragile states in the region.
Lebanon is torn by some of the same internal sectarian divisions as Syria and Jordan is struggling with its political reform path.
Iraq's prime minister, Nouri al-Maliki, warned on Wednesday that a victory by Syrian rebels would lead to more fighting in Iraq and a new haven for al-Qaida.
Greater instability in any of Syria's neighbors would pose a whole new set of problems.
Still, officials said the U.S. was considering a gradually upgraded involvement in Syria to bolster moderate forces within the rebel ranks and help the fledgling political opposition win greater backing among Syrians, especially minority groups that have remained largely loyal to Assad and his government.
Debate within the administration on how best to accomplish these goals has increased in recent months as diplomatic efforts have failed to end the war. The Syrian opposition insists that only weapons, intelligence support and other forms of military aid truly can tip the balance.
U.S. Sen. Marco Rubio, a potential 2016 presidential candidate, urged the administration to consider lethal aid.
"We should want the best organized, the best equipped and most dominant groups in the opposition to be groups that are friendlier to our national interests," Rubio, a Florida Republican, said Wednesday at the Washington Institute for Near East Policy.
The position is similar to one Kerry held as a senator, and one he reminded reporters of this week when he proposed the creation of opposition safe zones and suggested providing rebels with U.S. weaponry.
But in his first month as secretary of state and on his first official trip overseas, the 2004 presidential candidate has been vaguer.
"We have a lot of ideas on the table, and some of them, I am confident, will come to maturity by time we meet in Rome," Kerry said this week. "Others may take a little more of a gestation period, but they're no less part of the mix and part of the discussion.
"What I can tell you is we are determined that the Syrian opposition is not going to be dangling in the wind wondering where the support is or if it's coming, and we are determined to change the calculation on the ground for President Assad."
Klapper reported from Washington. Associated Press writers Kenneth Thomas in Washington, Sylvia Hui in London, and Silvie Corbet in Paris contributed to this report.
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