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

Jordan is the first of four training sites to begin the instruction. The others are in Saudi Arabia, Qatar and Turkey, and Turkish officials have been saying they expect the training there to begin this month.

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

There are about 450 coalition forces involved in the training at the four sites, including about 350 Americans.

Officials spoke on condition of anonymity because they weren't authorized to discuss the training publicly.

The U.S. has spent months vetting the fighters to try to make sure that any enemy combatants or extremists are weeded out. Pentagon officials have said it would be a very deliberative process that initially identified specific rebel groups and then moved to a lengthy vetting program that checks each fighter individually.

The rebel fighters, who come from several moderate groups in Syria, will get training on basic military equipment and skills, including firearms, communications and command and control abilities.

The U.S. military has been launching targeted airstrikes against the Islamic State group in Iraq since August, and expanded the campaign into Syria in September. The group has declared a self-styled Islamic state ruled by its strict religious views in territory it seized across much of Iraq and Syria, marked by a brutal campaign of mass murders, beheadings, torture and slavery.

Congress passed legislation authorizing the military to arm and train moderate Syrian rebels, providing $500 million for the U.S. to train about 5,000 fighters over the next year.