By Lesley Wroughton and Missy Ryan
WASHINGTON, June 1 (Reuters) - Since Syrians rose up more than three years ago against longtime President Bashar al-Assad, U.S. President Barack Obama has had a clear message: Assad must go.
Now, even as the United States seeks to increase support to moderate rebels to fight his regime, U.S. officials privately concede Assad isn't going anywhere soon.
The contrast between public rhetoric and private expectations reflects the Obama administration's struggle to address the increasingly complex, messy conflict in Syria, which is pitting world powers against one another - from Moscow to Tehran and Washington.
It also points to a continuation of the administration's policy of supporting Syria's neighbors and providing small-scale armed assistance to moderate rebels to fight the regime, while ruling out large-scale U.S. involvement that officials fear would lead to another Iraq or Afghanistan.
Assad's allies portray him as confident and in control ahead of a presidential election on Tuesday that the United States dismisses as a farce with the opposition largely unrepresented and unable to participate.
Obama said on May 28 he would work with Congress "to ramp up support for those in the Syrian opposition who offer the best alternative" to Assad and to extremists who could be more dangerous for the United States than Assad himself.
But despite that support, senior U.S. officials acknowledge in interviews the difficulty of removing Assad, who said in April the three-year war had swung decisively in his favor.
"I don't think anybody is under the impression that you're going to see a dramatic change in the near term in terms of the situation on the ground in Syria," said a senior U.S. official, who declined to be identified because of the sensitivity of the subject.
Details of the new U.S. assistance are largely unclear, but a proposed counterterrorism fund may help Syria's neighbors such as Jordan manage the flow of weapons, refugees and extremists out of Syria, a senior U.S. defense official said. The U.S. military may also train moderate rebels outside Syria, the officials said.
"The types of changes and programs that we are talking about are not intended to or likely to produce a dramatic change in immediate weeks," the senior U.S. administration official said.
Assad, with support from Russia, Iran and fighters from Lebanese militant group Hezbollah, has used tanks and warplanes to attack rebel-held areas. His opponents accuse him of using poison gas to force civilians in rebel areas to submission
Obama's Syria policy has faced intense criticism on multiple fronts, most notably for failing to follow through after declaring Syria's use of chemical weapons would cross a "red line" and be met with force.
As the conflict deepens, Syria's neighbors Jordan, Turkey, Iraq and Lebanon are struggling to contain the effects.
Lebanon is grappling with a refugee population that has increased the tiny country's population by a quarter; Iraq is beating back a surge in militant violence driven at least in part by Syria; and Assad's staying power has left Turkey's leadership frustrated and exposed.
"He may be elected to be president of Syria, but he won't control Syria," General Martin Dempsey, the top U.S. military officer, said in a recent interview with Reuters and the Pentagon's news service.
The scale of the suffering in Syria today would have been unimaginable in 2011, when the Arab Spring uprising toppled strongmen across the region and many Western officials felt confident Assad would meet the same fate.
More than 160,000 people have been killed and much of the country is in ruins. Some 2.7 million people, or more than 10 percent of Syria's population, have fled the country. Millions more are displaced within Syria.
The United States has provided more aid than any other country, providing more than $250 million in "non-lethal support" to opposition groups including communications kits and trucks.
But its steps to strengthen the rebels' military capacity have been less sure-footed. Last year, the administration began a small-scale covert program to train moderate rebel fighters in Jordan, but U.S. officials said they did not expect that alone would give rebel fighters a decisive edge.
Under the threat of U.S. airstrikes, Assad agreed with the United States and Russia in September to dispose of Syria's entire chemical weapons arsenal. It still possesses a significant amount of its declared chemical stocks and has not yet destroyed a dozen production and storage facilities.
International efforts to bring Syria's warring sides together to agree on a political transition have failed. Like others before him, Syrian mediator Lakhdar Brahimi quit out of frustration, blaming an international deadlock that hampered his ability to broker peace.
"The choice for the administration right now is whether to accept this sort of de facto uneasy partition of Syria, or to try to do something to roll it back," said Fred Hof, a former State Department special envoy for Syria who is now a senior fellow at the Atlantic Council's Rafik Hariri Center. (Additional reporting Patricia Zengerle; Writing by Missy Ryan, Editing by Jason Szep and Ross Colvin)