Defense Secretary Ash Carter said U.S. military troops have begun training a small, company-sized group of Syrian fighters, but adds that not "all of the rules of engagement" have been determined yet. (Reuters)

The U.S. military has launched a long-awaited program to train moderate rebels from Syria, Defense Secretary Ashton B. Carter said Thursday.

Carter, speaking to reporters at the Pentagon, said military officials are training about 90 Syrians at a foreign location, and that additional training would begin at a second site in a few weeks.

“This program is critical and a complex part of our counter-ISIL effort,” Carter said, using an acronym for the Islamic State, the extremist group whose militant domain straddles much of Syria and Iraq.

The training, which gets underway after months of delay, marks a new phase of the Obama administration’s reluctant involvement in Syria, where the political uprising of 2011 has morphed into a multi-sided civil war whose effects have spilled across the region.

Defense Secretary Ashton B. Carter, speaking to reporters at the Pentagon on Thursday, said military officials have begun training about 90 Syrians at a foreign location. (Mandel Ngan/AFP/Getty Images)

A U.S. official, speaking on the condition of anonymity to discuss activities in another country, said the initial training began Wednesday in Jordan. Mohammed al-Momani, a spokesman for the Jordanian government, confirmed that the training had started.

The program, which is separate from a CIA initiative to assist Syrian rebels, aims to train as many as 5,400 Syrians a year at facilities in Jordan, Qatar, Saudi Arabia and Turkey. Once they have undergone U.S. training, the fighters will be sent back into Syria with basic military gear, including ammunition, small arms, trucks and machine guns to mount on them.

Even as the training gets underway, the Obama administration and some of its allies view the goals of the program differently. The administration is seeking to produce a cohesive force to battle the Islamic State in Syria and, by extension, assist U.S. efforts to support an ongoing fight against the group in neighboring Iraq. Allied nations involved in the training, meanwhile, are more focused on supporting forces that will fight to weaken Syrian President Bashar al-Assad.

Officials struck a circumspect note as they announced the start of the program, which even some supporters worry is too small to make much difference on the ground. The Islamic State is only one of many armed groups that are fighting troops loyal to Assad, and each other. U.S. officials also acknowledge that they will have limited control over the fighters once they return to Syria equipped with American weapons and know-how.

“When President Obama unveiled the initiative nearly a year ago, he spoke of an anti-ISIL ground combat component that could also provide the basis for a negotiation process between the Assad regime and the Syrian opposition,” said Fred Hof, a former special adviser on Syria at the State Department. “Now, with maybe 2,000 people to be trained and equipped by the end of 2015, the administration correctly downplays the program’s import.”

The U.S. official said the Syrians being trained in Jordan hail from the same area of Syria and are being educated in basic military activities. The fighters are receiving a stipend from the United States.

The program is led by the Special Operations component of U.S. Central Command, and U.S. and Jordanian troops are conducting the training.

Military officials, who have expressed concern about telegraphing too much information about the fighters as they make their way back into Syria, declined to say exactly how long the training would last but said it would be weeks or months. It “will last as long as it takes,” the official said.

Numerous questions about future U.S. support for the new Syrian forces remain unanswered. Carter said the Obama administration had not yet decided whether the United States would commit to taking military action to protect the fighters if they come into conflict with forces loyal to Assad. Senior officials have been discussing this question for months.

“If they are contested by the regime, again, we would have some responsibility to help them,” Carter said. “We have not determined yet all of the rules of engagement.”

Carter said the Pentagon had taken extra measures to ensure the safety of U.S. trainers taking part in the program. In the past, U.S. soldiers have occasionally come under attack from foreign soldiers working alongside them or by militants dressed as soldiers.

Officials involved in the program have spent much of the past year developing an intensive vetting protocol including psychological evaluations and stress tests for the Syrians, who are coming out of a country where it is often difficult to distinguish between civilians and combatants.

The Pentagon will not have direct tactical control over the trained units once they deploy in Syria. “The biggest thing that’s happening here is that we’re establishing links” with the fighters, the U.S. official said, enabling the United States to exert indirect influence on Syria’s chaotic battlefield.

Taylor Luck in Amman contributed to this report.