The U.S. military announced Thursday it had begun a controversial program to train “moderate” Syrian rebels to combat the self-proclaimed Islamic State, even as it still are sorting out how the United States can protect such troops once they land on the Syrian battlefield.

At a joint press conference, Secretary of Defense Ashton Carter and Army Gen. Martin Dempsey, the chairman of the Joint Chiefs of Staff, said U.S. personnel had begun training 90 rebels who had been vetted and recruited months earlier. A second group of rebels will be trained in the next few weeks, creating the first U.S.-trained rebel company, Carter said.

They are among roughly 400 troops that the U.S. military has already vetted and recruited. Another 3,700 have expressed interest in joining the program, Defense officials have said.

While Carter would not say in which country the training was happening or when it began, a Jordanian official told the Associated Press the training began in his country “a few days ago.” Jordan was also the site of a CIA training program, which began in 2013, and since become a flailing effort.

With no U.S. ground forces in Syria and a nine-month U.S.-led air campaign that has done little to diminish ISIS’s strength, U.S. officials have long suggested that American-trained forces could be key to defeating the terror group.

A senior defense official said the forces would start by defending their own villages.

But the risks associated with the program are high—and the U.S. has not sorted out key details, Carter conceded.

The secretary could not specify how the United States would defend such troops should they come under attack from forces loyal to ISIS or Syrian President Bashar al-Assad. Carter said the U.S. would use drones and “possible air support help.”

“Of course we have some responsibility to protect forces,” Carter said.

Given that the U.S. is putting them into the fight, it will be obliged to defend such forces if they come under attack, much as U.S. has done for Iraqi security forces since the U.S.-coalition air campaign began in August. But where the Iraqi government forces number in the thousands and are backed with an infrastructure, a limited number of Syrian rebels will not have such support. And should they come under a complex attack, it will be difficult for U.S. air support to arrive in time, experts said.

“If you don’t have a significant mass of both manpower and equipment and training required to use that equipment effectively, there is a potential that these Syrian rebels could become more of a target for having affiliated with the United States,” Christopher Harmer, a senior naval analyst with the Middle East Security Project for the Washington, D.C.-based Institute for the Study of War, told The Daily Beast. “If we don’t defend them, our credibility is going to be nonexistent.”

The forces will receive a salary from the United States, Carter said, as well as small arms support. But it is unclear what the U.S. recourse would be should such troops commit war crimes. Carter said U.S. support was “strongly conditioned” in accordance with the rebels’ U.S.-training.

And there is the ongoing challenge of finding “moderate” forces in a four-year civil war, where one-time moderates have been pushed toward extremist groups.

“What made sense four years ago does not make sense now,” Harmer said.

How a few hundred rebel forces receiving basic training from U.S. forces could fend off attacks from both ISIS and Assad forces well versed in warfare remained unclear and for some, unlikely. Carter suggested the program could expand should the first tranche of troops succeed.

“You have to start somewhere and this is where we are starting,” he said.

Turkey and Saudi Arabia have joined forces to support anti-Assad forces, reportedly out of frustration with the slow-paced U.S. effort, signing a pact to create a rebel force. Turkey will reportedly provide ground forces backed by Saudi airstrikes. According to the Associated Press, the two countries will operate a joint command center in the northeast Syrian province of Idlib.

But the Saudis are currently struggling with their air campaign in Yemen, exposing the limitations of their military.