The two helicopters were from the nearby Kaneohe Bay US Marine Corps Air Station, and each had six people on board, CBS News reported, citing the Coast Guard.
Rescuers arrived to the scene just after midnight, after multiple reports of the crash, and found debris, an empty life raft and fire on the water, some 2.5km from the coastal settlement of Haleiwa.
No survivors or bodies have been located so far.
The search and rescue operation could be hampered by waves forecasted to reach up to 12 meters (40 feet) high - predicted to be the largest swell of this year's season.