HONOLULU, Jan 15 - The search for 12 Marines missing at sea after two helicopters collided near the island of Oahu in Hawaii was being hampered by high waves, U.S. Coast Guard officials said on Friday.

Two Coast Guard cutters and several Coast Guard aircraft were searching, along with two U.S. Navy warships and local police and fire department helicopters, Coast Guard spokeswoman Petty Officer Second Class Melissa McKenzie said.

"We remain hopeful" that survivors could be found, she said.

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 (1000 GMT), Coast Guard spokeswoman Sara Mooers said.

The wide-ranging search for the Marines was hampered by high surf and poor visibility from rain storms. The National Weather Service warned that a northwest swell would bring 35 foot to 45 foot surf to the area through Friday.

"It's very difficult to find things right now," Carr said.

Honolulu Emergency Services Department spokeswoman Shayne Enright confirmed in a telephone interview that visibility was not good.

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 spanned more than 7 miles off the coast, the Coast Guard said.

"Thoughts & prayers are with our Marines & 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, Coast Guard Lt. Scott Carr said at a news conference.