By Tom Perry and Suleiman Al-Khalidi

BEIRUT/AMMAN (Reuters) - On the eve of Donald Trump's election victory, members of a Western-backed Syrian rebel group met U.S. officials to ask about the outlook for arms shipments they have received to fight President Bashar al-Assad.

They were told the program would continue until the end of the year, but anything more would depend on the next U.S. administration, a rebel official at the meeting said. When Trump takes office in January, it may stop altogether.

The president-elect has signaled opposition to U.S. support for the rebels, and an overhaul of policy on Syria.

The military aid program overseen by the Central Intelligence Agency has given arms and training to moderate rebels in coordination with Turkey, Saudi Arabia, Qatar, Jordan and others.

It helped to support these rebels, fighting under the Free Syrian Army banner, as jihadist groups linked to al Qaeda emerged as a major force in a war approaching its sixth anniversary.

U.S. officials declined to comment on any meetings with rebel groups, and previously have not commented on the CIA program given its covert nature.

But Trump has indicated he could abandon the rebels to focus on fighting Islamic State which control territory in eastern and central Syria. He might even cooperate against IS with Russia, Assad's most powerful ally, which has been bombing the rebels for over a year in western Syria.

Assad, in an interview published on Tuesday, said Trump would be a "natural ally" if he decides to "fight the terrorists".

The rebels are looking on the bright side. They say support via the U.S.-backed program has been inadequate and Washington has stopped Saudi Arabia from giving them more powerful weapons.

So the rebels hope a more isolationist United States will give regional states a free hand, allowing Saudi Arabia to provide the anti-aircraft missiles President Barack Obama has vetoed.

The rebel official said there had been no contact with U.S. officials since Trump's win. But were U.S. support to end and "this veto lifted", that would be a good outcome, he said.

"Everybody is analyzing, there are positive expectations, there are negative expectations - but nothing is yet clear," the official said.

DARK DAYS FOR REBELLION

The prospect of a shift in U.S. policy comes at a dark time for the rebellion. Russia on Tuesday escalated its military campaign in support of Assad, drawing for the first time on an aircraft carrier it has sent to the region.

Assad and his allies are tightening their grip on rebel-held eastern Aleppo, where heavy air strikes have resumed and insurgents have failed to break the siege.

Longstanding tensions among rebels have turned into fighting twice in the Aleppo area this month.

But analysts also say it is too early to tell what Trump will do in Syria since his views could be reshaped by establishment thinking in Washington.

Republicans in his administration will not want to cooperate with Russia, or bow to the huge influence wielded by Iran in Syria, where thousands of Shi'ite militiamen including Lebanon's Hezbollah are fighting on Assad's side.

And to many in Washington, Assad remains anathema.

Yet since his election win, Trump has reiterated his misgivings about U.S. policy, telling the Wall Street Journal he "had an opposite view of many people regarding Syria" and "we have no idea" who the rebels are.

His comments cheered Damascus and its allies, which view his win as positive for their war effort.

"INTERNATIONAL COMMITMENT"?

"It is true that he doesn't know us, but the American state knows us and will tell him," said a second rebel leader whose group has been a recipient of military support. "There is an international commitment to us," he said. The rebels' other state backers were seeking to explain this to Trump, he said.

Western policy towards Syria has been built around the idea that there can be no sustainable peace with Assad in power.

Assad, a member of the minority Alawite sect, is dependent on military support from Russia, Iran and Shi'ite Islamist militias in the fight with the Sunni Muslim insurgency.

Western policymakers believe the nationalist Sunni rebels are needed to build a stable Syria.

But their policy has long been hampered by splits in the opposition and the prominent role jihadists have played in the insurgency. A Western diplomat said jihadist influence would increase were Trump to abandon the FSA rebels.

In western Syria, FSA rebels have often fought in close proximity to jihadists against the army and its allies.

Concerns about weapons ending up in jihadist hands still appear to act as a brake on military support to the rebels.

With the collapse of a ceasefire brokered by the United States and Russia in September, U.S. officials considered military options including direct U.S. military action such as air strikes on Syrian military installations.

But rebels say there has been no big shift since then.

Were the United States to abandon the rebels, their military fortunes would hinge on Saudi, Qatari and Turkish support. Officials from those countries could not immediately be reached for comment on the subject of their backing for the rebels.

The rebels believe Turkey for one remains a steadfast backer. But its recent rapprochement with Russia has raised questions over Turkish aims in Syria. Ankara appears more set on rolling back Kurdish influence and Islamic State than getting more deeply involved in the war for Aleppo, for example.

The Syria conflict has killed hundreds of thousands of people, and divided Syria into areas controlled by the government, insurgent groups, Kurdish militia, and Islamic State.

The Kurdish YPG militia is at the center of U.S. strategy for fighting Islamic State in Syria, despite opposition from U.S. ally Turkey, which fears Kurdish influence in northern Syria will fuel separatism among its Kurdish minority.

The Pentagon also backs some Syrian Arab rebels fighting Islamic State, despite the failure of a program last year which only trained a few dozen fighters.

The spokesman for one such group, the New Syria Army, forecast reduced U.S. support for the rebels as Trump sought to "understand the picture more and to separate the jihadist groups from the moderate groups".

But in the end, U.S. policy will be forced to "support the FSA groups that have a nationalist complexion", said the spokesman, Muzahim Saloum.

Mohamad Aboud, an ex-rebel commander and a member of the main opposition political body, the High Negotiations Council, said Turkish influence would help shape a more supportive U.S. policy towards the rebels.

Unlike with Obama, there would "be clarity in the new Trump administration", he said.

(Writing by Tom Perry; editing by Giles Elgood)