A Syrian rebel force of 12,000 to 15,000 would be needed to take territory held by Islamic State militants in the country's east, three times the number of fighters due to be trained by the United States, the top US military officer said Friday.

"We've had estimates anywhere from 12,000 to 15,000 is what we believe they would need to recapture lost territory in eastern Syria," General Martin Dempsey told a news conference at the Pentagon.

 But he said the 5,000 recruits scheduled to be trained and armed by American instructors was never intended to represent the total number of troops for the "moderate" rebel forces.