The U.S. force deploying to the Middle East later this year to train and equip moderate Syrian rebels may exceed 1,000, a Pentagon spokesman said Friday.

The mission will involve about 400 U.S. trainers — likely to include many Army Special Forces personnel — as well as additional "enablers" who will provide security and logistical support for the trainers, defense officials say.

"It could approach 1,000, it could even exceed it," said Rear Adm. John Kirby, a Pentagon spokesman, in a briefing with reporters Friday.

The training will begin as early as this spring, and the American trainers and support troops will work out of sites in Saudi Arabia, Turkey and Qatar, Kirby said.

No American troops have received deployment orders yet, but that could begin "in the next week or so," Kirby said. Those troops will start arriving at the training sites in Turkey, Qatar and Saudi Arabia in about four to six weeks, he said.

At least two other partner nations will send trainers for the mission, Kirby said. He declined to identify the countries.

The U.S. plans to train moderate Syrian rebels in neighboring countries and then send those rebel forces back to Syria to protect their homes and to fight the militants known as the Islamic State group.

Simultaneously, the U.S. has sent more than 2,000 U.S. troops into Iraq to help train, advise and assist the Iraqi military's fight against the same group. President Obama has authorized the deployment of up to 3,100 U.S. troops to that country.

The effort to train Syrian rebels is part of the overall campaign to defeat the Islamic State militants who seized large swaths of Syria and Iraq last year.

U.S. troops have been deploying into Iraq to train security forces there, including from the 1st Infantry Division and the 82nd Airborne Division's 3rd Brigade Combat Team.

Training in Iraq has begun in areas including Taji, north of Baghdad, and Al Asad Air Base, in western Iraq.

Staff writer Michelle Tan contributed to this report.