National Security Adviser Susan Rice told reporters that the administration will move as fast as it "reasonably can" after Congress approved the president's plan to train and arm Syrian rebels, Xinhua reported.
"But this is something that will take many months," she said at a White House daily news briefing.
President Barack Obama said the training programme will be implemented outside of Syria in partnership with regional countries. Rice said the training facilities need to be constructed and the trainees need to be brought in for training.
Training programme will be implemented outside of Syria
"It is not something that one should expect will yield rapid and immediate fruit," Rice said, adding, "This is a serious training programme, and we're serious about vetting those that we will be training and equipping."
Obama has pledged not to send US troops into combat against the IS, but rely on partner forces on the ground to support air raids carried out by US warplanes.
The administration is reportedly aiming for training about 5,000 what it calls "moderate" Syrian rebels in support of upcoming US airstrikes inside of the Arab country.
Obama had authorised strikes on the IS in Syria, but Rice refused to say when the president would give the go-ahead.