Personnel from the U.S. Coast Guard, Navy, and Honolulu Fire Department looked for survivors seven miles offshore in increasingly poor weather conditions. Swells reached 16 feet, surf rose to 40 feet and visibility extended to only one mile, the Coast Guard said.
"The weather is making it very difficult," Coast Guard spokesman Lt. Scott Carr told reporters. "It is very difficult to find things right now.
"Our goal is to find survivors," he said.
Loved ones have identified to CNN three Marines missing in the crash, and a CNN affiliate has reported on a fourth missing Marine.
Anthony Kuenzel told CNN that his brother-in-law, Capt. Kevin Roche, is among them. The family described him as a passionate Marine and thanked people praying for his safe return.
"We believe the Marines and Coast Guard are doing everything they can to bring Kevin and his fellow Marines home safely and we are grateful to everyone involved in the rescue," Roche's family said in a statement.
Maj. Shawn Campbell, 41 was in the collision, said his mother, Donna McGrew, of Houston. A Marine officer visited her at home to brief her. Campbell has a wife and four children, and has done three tours of duty in the Middle East.
"My heart just breaks, there are 11 other families going through this now. God bless them," McGrew said.
Another Texan, Cpl. Matthew Drown, was on board one of the helicopters and is missing, his brother Sean Drown told CNN.
Cpl. Drown, 23, has been in the Marines for five years, and joined shortly after high school.
The search is taking place off the north shore of Oahu, a popular surfing destination. Surfers should avoid any debris in the water, Carr said. Searchers have already spotted floating pieces of debris consistent with military aircraft.
"The Coast Guard is asking that people use extreme caution along the coastal area surrounding the wreckage site which stretches from Mokule'ia Beach to Turtle Bay," Carr said in a statement.
"We are urging people to stay out of the water and off the beaches due to debris that could pose potential risk and cause serious bodily harm," he added.
Marine Commandant Gen. Robert B. Neller expressed hope.
"Thoughts & prayers are with our Marines & their families in Hawaii as search efforts continue. We remain Semper Fidelis," Neller tweeted.
Earlier, searchers spotted a fire and debris field, including an empty life raft, about 2½ miles north of Haleiwa Beach, according to Coast Guard Chief Petty Officer Sara Mooers.
The two helicopters were believed to have collided late Thursday, Mooers said. The Coast Guard received notice at 11:38 p.m. local time.
The two aircraft apparently didn't issue a mayday call, Carr said.
Rather, a civilian on the beach first notified authorities of seeing a fireball, followed by a second person reporting a flare, Carr said.
Marine officials contacted the Coast Guard about the missing aircraft around 11 p.m. (4 a.m. Friday ET), Mooers said. A Coast Guard helicopter arrived shortly after midnight (5 a.m. ET), according to the agency.
CNN affiliate KHON
in Hawaii reported that witnesses said they heard a loud boom over the water and saw a flash in the sky.
Six people were on board each of the two heavy-lift transport helicopters, according to Mooers.
The missing aircraft are assigned to Marine Heavy Helicopter Squadron 463, Marine Aircraft Group 24, 1st Marine Aircraft Wing, the Marines tweeted.
According to the unit's website, it is stationed at Marine Corps Naval Air Station Kaneohe Bay, on the opposite side of the island from where the apparent collision occurred.
Participating in the search Friday were two Navy warships, Coast Guard Cutters Ahi and Kiska, a MH-65 Dolphin helicopter, a HC-130 Hercules airplane, a MH-60 helicopter, Honolulu Fire Department crews, and a helicopter rescue boat, officials said.
A high surf advisory is in effect for the north facing shore of Oahu where the search is being conducted, authorities said.
The Marine Corps are investigating the cause of the accident.