High waves for the next several days will hamper the search for 12 Marines missing at sea after two helicopters collided near the island of Oahu in Hawaii, US Coast Guard officials say.
Two Coast Guard cutters and several Coast Guard aircraft were searching, along with two US Navy warships and local police and fire department helicopters, the Coast Guard said on Friday.
A safety zone has been set up from the shoreline that matches up with the accident site to 13 kilometres out to sea, the Coast Guard said.
"We've seen debris through the entire area," said Lieutenant Scott Carr, a Coast Guard spokesman. He added the high surf was expected to last until at least Tuesday.
The CH-53E helicopters, belonging to the 1st Marine Aircraft Wing from the Marine Corps Air Station at Kaneohe Bay, were reported to have collided just before midnight local time, Coast Guard spokeswoman Sara Mooers said.
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The wide-ranging search for the Marines was hampered by high surf and poor visibility from rainstorms. The National Weather Service warned that a northwest swell would bring 11-metre to 14-metre surf to the area.
"It's very difficult to find things right now," Carr said.
A Coast Guard helicopter crew spotted debris in the water off the town of Haleiwa on the north shore of Oahu but they did not find passengers.
The debris field extended more than 11km off the coast, the Coast Guard said.
"Thoughts and prayers are with our Marines and their families in Hawaii as search efforts continue," General Robert Neller, commandant of the Marine Corps, said in a message on Twitter.
No distress call was issued by either aircraft. Authorities were notified by a man standing on the beach who saw a fireball over the ocean after seeing the helicopters flying in that area, Carr said at a news conference.