Two US military helicopters carrying six passengers each, are reported to have collided off the northern shore of Oahu, Hawaii on Friday morning.
Rescue workers have begun a search operation for the missing 12 people in a debris field 2 miles off the coast of Haleiwa.
"They are currently searching for any possible personnel," said spokeswoman for the Coast Guard Sarah Mooers.
The US Navy and a local fire department is aiding the search, which has been hampered by dark, cloudy conditions and large waves of up to 15 feet, she said.
Officials of the Marine Corps were not immediately available for comment.
There was no immediate word on what caused the collision or if any survivors have been found.