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 yesterday 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."
The US Congress backed legislation last month to fund the training and equipping of Iraqi forces and moderate Syrian rebels, allocating about USD 500 million for the Syrian effort.
But President Barack Obama has been accused of moving too slowly to help the opposition while the IS 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 the moderate rebels at arm's length. But the rise of the IS group last year prompted a change in strategy, with the US president announcing plans in September to train and arm the opposition.