WASHINGTON/REYHANLI, TURKEY –

Hundreds of Syrian rebels will soon start U.S. training to battle the Islamic State group without knowing whether or how Washington would come to their aid on the battlefield.

The U.S. plan to train and arm a force that is expected to eventually total more than 15,000 troops and to get underway in the coming weeks is a major test of President Barack Obama's strategy of engaging local partners to combat extremists.

But administration officials are already scaling back expectations of its impact. And some rebel leaders say the force risks sowing divisions and cannot succeed without directly targeting Syrian forces.

Senior U.S. officials said Obama has not yet decided how extensively and under what circumstances Washington will back the force militarily — a commitment that could bring an entanglement in Syria that Obama has long sought to avoid. Senior U.S. military officials say protecting the forces will be vital to drawing new recruits and ensuring the success of the program.

The hard-line Sunni Muslim Islamic State movement has seized swaths of Syria and Iraq and proclaimed a caliphate.

In the face of its brutal offensive through northern Iraq last June, Obama asked Congress for an initial $500 million to "train and equip" Syria's opposition fighters, whom he later described as "the best counterweight" to Islamic militants. The total request so far is $1.1 billion.

Ten months later, it is just getting off the launchpad.

U.S. officials say several hundred U.S. and coalition personnel are preparing to start training, initially at sites in Jordan and Turkey and later in Saudi Arabia and Qatar. The sessions, which were originally expected to start in March, will cover everything from the rules of war to small-arms skills.

The plan reflected the priorities of a president who is reluctant to get entrenched in another Middle East conflict but who needed a ground force to complement U.S. airstrikes against the Islamic State group in Syria. Congress approved it last year with two primary goals: to fight the group and to boost the chances of a negotiated settlement in the Syrian war by raising pressure on President Bashar Assad's forces.

Opposition fighters and some of their backers, including Turkey and Saudi Arabia, say the plan will only succeed if it focuses more directly on fighting Assad's forces and allied militias.

"For Syrians, the biggest motive is to stop the killing, and the biggest killer is the regime," said Abu Hamoud, head of operations at Division 13, a non-Islamist opposition brigade that has received training under a separate CIA-led program.

Part of the U.S. strategy, according to Obama administration documents seen by Reuters, is to pressure Assad by steadily increasing the opposition's prowess and territory under control.

But the documents acknowledge that the U.S.-trained rebel force's impact is likely to be modest — at least initially.

Obama has also yet to announce whether he will go beyond resupplying and financing the proxy force and protect them with U.S. fighter jets if they clash with Assad's forces. The United States is already conducting near-daily airstrikes against Islamic State targets in Syria and Iraq.

About $143 million of this year's $500 million budget for the proxy force has been spent on weapons, ammunition, equipment and other items, the Pentagon says.

A Pentagon report to Congress suggested the training mission could expand greatly this year but would be conducted by "fewer than 1,000" U.S. military personnel.

The goal of the program is to train up to 5,400 recruits a year, the Pentagon says.

Already, more than 3,000 Syrian candidates have volunteered for the program and are in various stages of screening and vetting, it says.

Once they are back fighting in Syria, the rebels "will require persistent replenishment of critical supplies," the Pentagon report says.

The Pentagon plans to resupply them with equipment and munitions, and provide rebel leaders with cash stipends.