President Obama's $500 million plan to raise an army of Syrian rebels to fight the Islamic State horde is falling far short of its target, attracting only a few dozen recruits to the cause. 

"As of July 3, we are currently training about 60 fighters," Defense Secretary Ash Carter told lawmakers Tuesday.

He and Chairman of the Joint Chiefs of Staff Gen. Martin Dempsey faced skeptical lawmakers on the Senate Armed Services Committee as they addressed the administration's strategy for fighting the Islamic State. Even after Obama said a new effort to train Iraqi fighters is starting to pick up, Carter's estimate showed how far behind they are on the Syrian side of the border. 

"That's not a very impressive number," committee Chairman Sen. John McCain, R-Ariz., told Carter, after the Pentagon chief revealed just 60 Syrians are in training. 

White House Press Secretary Josh Earnest, asked by Fox News about the training numbers, also said Tuesday afternoon that 60 trainees "is clearly not enough," but added the vetting process is critical and takes time. 

To date, fewer than 100 Syrians reportedly have entered training as part of a mission Obama has sought $500 million for over three years. 

The training, however, only got started in May after months of recruiting and vetting of volunteers. The stated U.S. goal is to train and equip 5,400 rebels per year, and military officials said recently they still hope for 3,000 by year's end. 

How they'll get there is unclear. 

Carter said Tuesday they're in the "early stages" of the train-and-equip mission, and are working to screen and vet nearly 7,000 volunteers in Syria. 

The process, he said, involves a "counterintelligence screening" and a check to make sure any recruits "meet standards prescribed by U.S. law." 

The Syria training mission has been one of the trickiest components of the U.S. strategy against ISIS. 

The main problem thus far has been finding enough Syrian recruits untainted by extremist affiliations or disqualified by physical or other flaws. 

"We have set the bar very high on vetting," Col. Steve Warren, a Pentagon spokesman, said last month. 

In Syria, the U.S. and the rebels also have dual enemies -- the Islamic State and the Assad regime. 

Carter stressed Tuesday that "we are telling them that we are arming and training them in the first instance to go after ISIL and not the Assad regime. That's our priority." 

That program, together with a more advanced but also troubled parallel effort to rebuild the Iraqi army, is central to the U.S.-led effort to create ground forces capable of fighting ISIS without involving U.S. ground combat troops. 

Obama, after meeting with military officials at the Pentagon, said Monday that the battle will be a "long-term campaign" but vowed ISIS would be defeated in the end. 

Fox News' Lucas Tomlinson and The Associated Press contributed to this report.