By Jonathan Landay and Arshad Mohammed

WASHINGTON, Sept 26 (Reuters) - The collapse of the latest Syria ceasefire has heightened the possibility that Gulf states might arm Syrian rebels with shoulder-fired missiles to defend themselves against Syrian and Russian warplanes, U.S. officials said on Monday.

Still, the U.S. administration continues to maintain that negotiations are the only way to end the carnage after Russian-backed Syrian forces intensified their bombing of Aleppo, the last major urban area in rebel hands.

The latest U.S. attempt to end Syria's 5-1/2 year civil war was shattered on Sept. 19 when a humanitarian aid convoy was bombed in an attack Washington blamed on Russian aircraft. Moscow denied involvement.

On Monday, medical supplies were running out in rebel-held eastern Aleppo, with victims pouring into barely functioning hospitals as Russia and its Syrian ally President Bashar al-Assad ignored Western pleas to stop the bombing.

One consequence of the latest diplomatic failure may be that Gulf Arab states or Turkey could step up arms supplies to rebel factions, including shoulder-fired anti-aircraft missiles, something the United States has largely prevented until now.

One U.S. official, speaking on the condition of anonymity to discuss American policy, said Washington has kept large numbers of such man-portable air defense systems, or MANPADS, out of Syria by uniting Western and Arab allies behind channeling training and infantry weapons to moderate opposition groups while it pursued talks with Moscow.

But frustration with Washington has intensified, raising the possibility that Gulf allies or Turkey will no longer continue to follow the U.S. lead or will turn a blind eye to wealthy individuals looking to supply MANPADS to opposition groups.

"The Saudis have always thought that the way to get the Russians to back off is what worked in Afghanistan 30 years ago - negating their air power by giving MANPADS to the mujahideen," said a second U.S. official.

"So far, we've been able to convince them that the risks of that are much higher today because we're not dealing with a Soviet Union in retreat, but a Russian leader who's bent on rebuilding Russian power and less likely to flinch," this official said, referring to Russian President Vladimir Putin.

Asked if the United States was willing to do anything beyond negotiations to try to stop the violence, State Department spokesman Mark Toner did not outline other steps, but stressed that Washington does not want to see anyone pouring more weapons into the conflict.

"What you would have as a result is just an escalation in what is already horrific fighting," Toner said. "Things could go from bad to much worse."

RIGHT TO DEFENSE

Another administration official, however, said, "The opposition has a right to defend itself and they will not be left defenseless in the face of this indiscriminate bombardment."

Speaking on condition of anonymity, the official noted that other U.S. "allies and partners" have been involved in the U.S.-Russian talks to find a resolution to the war. "We don't believe they will take lightly to the kind of outrages we've seen in the last 72 hours," said the administration official, who added that he would not comment on "the specific capability that might be brought into the fight." He declined to elaborate.

Critics of U.S. President Barack Obama, who has sought to avoid getting into another Middle East war and seems unlikely to do so in his final months, argued that U.S. diplomacy has been hamstrung by the White House's reluctance to use force.

"Diplomacy in the absence of leverage is a recipe for failure," Senators John McCain and Lindsey Graham, Republican critics of the Democratic White House, said in a statement.

"Putin and Assad will not do what we ask of them out of the goodness of their hearts, or out of concern for our interests, or the suffering of others. They must be compelled, and that requires power," they added. "Until the United States is willing to take steps to change the conditions on the ground in Syria, the war, the terror, the refugees, and the instability will all continue."

"BEYOND THE PALE"

White House spokesman Josh Earnest accused the Russians of targeting the civilian water supply of eastern Aleppo used by refugee camps, aid convoys, and the White Helmets, a civilian group that seeks to rescue victims of air strikes.

"The idea of weaponizing access to a clean water supply for civilians; it's beyond the pale," Earnest told reporters.

Sarah Margon, director of Human Rights Watch's Washington office, said the actions alleged by Earnest "all constitute war crimes under international law."

"The U.S. has treated Putin as a partner in peace instead of an accomplice and perpetrator of war crimes," Margon said. "The question is now what steps the U.S. will take to compel Russia to refrain from further abuse and from facilitating Assad's atrocities."

The White House did not immediately respond to an emailed question on whether the United States believed that Russia has committed war crimes, a charged made by Britain. (Additional reporting by Ayesha Rascoe in Washington and Patricia Zengerle in Cartagena, Colombia; Writing by Arshad Mohammed; Editing by Andrew Hay)

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