Despite years of diplomacy and a CIA operation to vet and train moderate rebels, the U.S. finds itself without a credible partner on the ground in Syria as it bombs the Islamic State group. That's a potentially serious flaw in its strategy to ultimately defeat the militants.

Obama administration officials have long conceded that airstrikes alone won't drive IS from its strongholds across Syria and Iraq, but it also has ruled out the use of American ground troops. The U.S. strategy to crush IS rests on the use of local proxy forces, and hinges on plans to use $500 million and a base in Saudi Arabia to build an army of moderate Syrian rebels.

The ground force component has always been seen as a challenge in Syria, but the difficulty has become clearer in recent days. Officials acknowledge that the U.S. doesn't trust any Syrian rebel groups enough to coordinate on the air campaign, despite attempts by some pro-Western fighters to pass along intelligence about IS positions.

The CIA has secretly trained and is paying more than 1,000 moderates to help achieve the administration's stated objective of overthrowing Syrian president Bashar Assad, U.S. officials have said.

Those fighters have been gaining ground against Assad in southern Syria and in some places are fighting IS, said Robert Ford, a former U.S. ambassador to Syria. The CIA-funded fighters have proven reliable and have made modest gains, said a congressional aide who has been briefed on the matter. The aide spoke only on condition of anonymity to discuss sensitive intelligence.

But some analysts have questioned the fighters' loyalty and competence. Either way, it's clear their impact has not been decisive.

"Most of these groups have worked closely with Jabat al Nusra at some point in the last year or so," said Joshua Landis, the Arabic-speaking director of the Center for Middle East Studies at the University of Oklahoma, referring to the head of Syria's al-Qaida spinoff. "Some of them have worked hand in glove with ISIS. For Americans to call a sit-down and say 'Here's where we're bombing' doesn't make any sense. We don't trust these guys."

American officials don't go that far in public remarks, but they have been fairly blunt.

"We don't have a willing, capable, effective partner on the ground inside Syria right now," Rear Adm. John Kirby, the Pentagon spokesman, said last week. "It's just a fact."

John Allen, the retired Marine general in charge of coordinating the U.S.-led coalition against the Islamic State group, told reporters Wednesday that "at this point, there is not formal coordination with" the U.S.-backed moderate rebels known as the Free Syrian Army.

That approach has infuriated rebels, fueling mistrust on both sides. The commander of a moderate rebel brigade in the northern Aleppo province, who goes by the nom de guerre Abu Thabet, called the U.S.-led airstrikes "pointless and self-serving."

As Americans have bombed IS positions elsewhere, Syrian government forces have advanced in northern Aleppo province, Abu Thabet said. Moderate factions like his are trapped between IS fighters on one side and government forces on another, and the U.S. has not once hit IS along the 12-mile front it occupies against his group, he said.

Abu Thabet said rebels have tried to pass along information about IS positions to the U.S. military, but have received no response.

"The Americans are kidding themselves," he said. He then praised the Nusra Front — underscoring the sort of concerns that bedevil U.S. policymakers.

"I am surprised at how fractious and disunified the Syrian opposition has been," said Michael O'Hanlon, a military strategy expert at the Brookings Institution. "They just haven't managed to find a charismatic leader or a single rallying point."

Part of the explanation, he and others said, rests with the decision by the Obama administration not to fund and equip the moderates three years ago, before Nusra and IS grew in strength.

Allen said the U.S.-led coalition intends "to build a coherence to the Free Syrian Army elements that will give it the capacity and the credibility over time to be able to make its weight felt in the battlefield against ISIL. It's going to require a build phase. It's going to require a training and equipping phase."

But critics question whether $500 million and several thousand fighters will be enough.

"I do not understand how 5,000 to 10,000 men are going to hold the eastern half of Syria," said Ford, the former ambassador. "It looks woefully inadequate to me."

O'Hanlon, the analyst, added that the numbers suggest the Obama strategy is "not that serious."

Rep. Mike Pompeo, a Kansas Republican and former Army officer who serves on the House intelligence committee, said he heard during a just-concluded trip to Saudi Arabia, Lebanon, Jordan and Turkey a widespread "fear that the Americans' stated objectives aren't consistent with our actions to date."

Pompeo, Ford, O'Hanlon and many other observers believe that the Obama administration ultimately will have to take tougher action against the Assad government, perhaps including a no-fly zone, to induce Syrian Sunni Arabs to fight IS. Carl Levin, the Michigan democrat who chairs the Senate Armed Services Committee, called on Thursday for a no-fly zone and a buffer zone to protect civilians. The Syrian foreign ministry said Thursday it was "ferociously opposed" to any such measures.

"We need a no-fly zone," Pompeo said, "because each time (the moderates) begin to make some progress, they get barrel bombed."

___

Karam reported from Beirut. AP's Bassem Mroue contributed from Beirut.

Follow Ken Dilanian on Twitter at https://twitter.com/KenDilanianAP