WASHINGTON -- After months of delays and vetting, the training of moderate Syrian rebels has started in Jordan as part of a broader effort to build a force capable of fighting Islamic State extremists, U.S. and Jordanian officials said Thursday.

Defense Secretary Ash Carter acknowledged, however, the administration has yet to decide exactly how and when the U.S. would support or go to the rescue of Syrian rebel troops if they are attacked by forces under the control of President Bashar Assad.

Carter said the U.S. is training the rebel troops to fight against Islamic State militants and will support the rebels in that effort with surveillance or airstrikes. But he said while the U.S. bears some general responsibility to protect the rebels, there have been no decisions yet on what the U.S. would be willing to do if the trained rebels return to Syria and get into a battle with regime troops.

Calling the training program a critical and complex part of the campaign to defeat the Islamic extremists, Carter said the initial segment of the training program will involve about 90 Syrian rebel fighters.

Gen. Martin Dempsey, chairman of the Joint Chiefs of Staff, added the Assad regime's momentum against the rebels has slowed, and any dramatic change of rule in the country could increase the humanitarian crisis there.

Fighting has intensified in several parts of Syria in recent weeks as rebel advances have pushed out government forces from contested areas in the country's south and north. Government troops have been pushing back.

Jordan is the first of four training sites to begin the instruction. The others are in Saudi Arabia, Qatar and Turkey. Turkish officials have said the training would start there this weekend.

More than 3,750 Syrian fighters have volunteered for the training, and about 400 have completed the prescreening. U.S. officials have said each training class could have up to 300 participants.