The American troops will be deployed starting in early spring on six- to eight-week missions in three countries -- Turkey, Qatar and Saudi Arabia -- as part of the Obama administration's plan to expand training for moderate Syrian rebels.
Those groups are battling forces loyal to Syrian President Bashar al-Assad and extremist groups like ISIS, as well as others linked to al Qaeda.
The mission would be led by the military and complement a modest CIA training program, based on what was authorized by the White House more than a year ago.
"Saudi Arabia, Turkey and Qatar have agreed to host training sites, and we anticipate the program to train and equip the moderate Syrian opposition will take approximately 400 U.S. trainers, with the caveat that this number could fluctuate based on course load, course requirements and other variables," said Cmdr. Elissa Smith, a Pentagon spokeswoman. "We also anticipate that the trainers will have enablers. We expect to begin training in early spring."
They trainers focused on Syria will join up to 3,000 American troops that were authorized by President Obama last year to act as anti-ISIS fighters in Iraq, which shares a border with Syria. American involvement in the battle against ISIS has been more robust in Iraq, where the government has requested assistance. American planes have also conducted numerous bombing missions in both countries.
This new training program is meant to help "moderate Syrian fighters" target ISIS. The moderate fighters are also at war against al-Assad, whom the Obama administration also wants to see defeated. But in her statement, Smith didn't mention al-Assad as a target of the U.S. training mission.
"The goal for the train and equip program is to build the capabilities of the moderate Syrian fighters to defend the Syrian people; stabilize areas under opposition control; promote the conditions for a negotiated settlement of the conflict in Syria; and empower trainees to go on the offensive against ISIL," she said, using the U.S. government term for ISIS.
U.S. officials have also said that vetting the rebels who take part in the program will play an important role to assure they do not share any dual allegiances to Jabbat al-Nusra, al Qaeda's affiliate in Syria, or any other militant groups.
Rear Adm. John Kirby, the Pentagon press secretary, has said training the vetted rebels could take eight to 12 months before they are ready to return to Syria for the fight against ISIS.
CNN's Chloe Sommers, Z. Byron Wolf and Jamie Crawford contributed to this report.