The training will take place in Saudi Arabia, Turkey and Qatar and is expected to start in the "early spring," spokesman Rear Admiral John Kirby told a news conference.
More than 400 trainers would start deploying in the next four to six weeks, backed by a similar number of support troops that will provide help with logistics, communications and intelligence, he said.
The total troop number "for this mission could approach 1,000," Kirby said. "It might even exceed that."
About 5,400 Syrian rebels would be trained and armed in the first year of the program, with an initial group of fighters possibly ready to enter battle "before the end of the year," he said.
The US Congress backed legislation last month to fund the training and equipping "moderate Syrian rebels", allocating about $500 million for the Syrian effort.
But President Barack Obama has been accused of moving too slowly to help the opposition while the ISIS group and other hardline extremists have taken on an increasingly dominant role on the battlefield.
Obama, wary of having the US drawn into Syria's multi-sided civil war, had long kept what he called “moderate rebels” at arm's length. But the rise of the ISIS group last year prompted a change in strategy, with the US president announcing plans in September to train and arm the opposition.
The US administration, however, has placed a priority on fighting the ISIS militants rather than President Bashar al-Assad's government, an emphasis that has led to disagreements with some allies, including Turkey.
The Pentagon made no mention of Assad in announcing the training effort.
The American troops will complement a small CIA training program that is already in place but officials declined to discuss whether the military would oversee the spy agency's efforts.
Two other countries also have pledged to help train the Syrian fighters, but officials did not say which governments or how many trainers they would contribute.
US intelligence agencies and foreign partners are working to identify potential recruits for the training program, who will then be vetted to ensure they have no ties to the ISIS group or other extremists, officials said.
Major General Michael Nagata, who is overseeing the US-led fight against the ISIS terrorists in Syria, met a range of opposition figures in Istanbul earlier this week and came away encouraged, Kirby said.
To avoid a repeat of insider attacks that have plagued the military training mission in Afghanistan, a portion of the US troops to be deployed would focus on security for their compatriots, he said.
In Iraq, US troops already have started training Iraqi government and Kurdish forces in the fight against the IS jihadists, with about 2,100 Americans deployed there.