(CNN) -- Five US Marines who were missing after two aircraft collided mid-air off the coast of Japan on Thursday morning have been declared dead, the US Marine Corps said.
The US military called off its search for the Marines on Tuesday, according to a statement from the Marine Expeditionary Force.
"The Marine Corps has pronounced the five remaining Marines involved in the F/A-18 and KC-130 aviation mishap deceased," the Marine Corps said in a statement. "The change in status comes at the conclusion of search and rescue operations."
The identities of the Marines have not been released, but the Marine Corps said their next of kin have been notified.
"Every possible effort was made to recover our crew, and I hope the families of these selfless Americans will find comfort in the incredible efforts made by the US, Japanese, and Australian forces during the search," Marine Corps Lt. Gen. Eric Smith, commander of the III Marine Expeditionary Force, said in the statement.
US, Japanese and Australian forces conducted more than 800 hours of air and maritime search operations, covering more than 35,000 square miles of ocean, US Forces Japan said in a statement.
The five Marines were among seven crew aboard two aircraft -- a KC-130 and F/A-18 -- that collided Thursday at 1:42 a.m. local time about 200 miles (320 kilometers) off the coast of Japan. The planes had launched from Marine Corps Air Station Iwakuni, the Marine Corps said in a statement.
The death toll from the accident now stands at six. Two Marines were rescued following the crash. One of them, Capt. Jahmar F. Resilard, 28, later died, the Marine Corps said.
The Marine Corps initially said the accident occurred during an aerial refueling that was part of routine training, but on Tuesday said it is "not confirmed" that refueling was ongoing when the collision occurred. The primary mission of a KC-130 is airborne refueling.
The cause of the crash is under investigation.
The 7th Fleet
The two planes belonged to the US Marine Corps, assigned to the 1st Marine Aircraft Wing, III Marine Expeditionary Force.
While the cause of Thursday's crash is under investigation, it comes a year after a spate of incidents -- most involving ships assigned to the US 7th Fleet -- that included the deaths of 17 sailors.
In August 2017, Vice Adm. Joseph Aucoin was relieved of his duty as the commander of 7th Fleet following a deadly collision between the destroyer USS John S. McCain and a merchant ship off Singapore that left 10 US sailors dead.
In June the same year, the guided-missile destroyer USS Fitzgerald, also of the 7th Fleet, collided with a cargo ship off Japan, killing seven US sailors.
Those two fatal collisions were "avoidable" and "numerous failures occurred on the part of leadership," a Navy report concluded November last year.
Appearing before the House Armed Services Committee as part of an investigation into the series of fatal crashes and collisions at sea, the Navy's No. 2 officer, Vice Chief of Naval Operations Adm. William Moran, said the Navy was trying to do too much with too little.
"We continue to have a supply-and-demand problem which is placing a heavy strain on the force," said Moran. "All of these things culminate with this notion we aren't big enough to do everything we're being tasked to do."
Surveys of sailors aboard the Japan-based guided-missile cruiser USS Shiloh carried out between June 2015 and August 2017 pointed to severe morale problems.
One sailor described serving aboard the ship as being akin to being on "a floating prison," according to surveys obtained via the Freedom of Information Act by Navy Times.
In the wake of the incidents, the US Navy began reevaluating its operations and included efforts to improve conditions for sailors and personnel, according to the Navy Times in April.
Still, accidents have continued. Most recently, a US Navy helicopter crashed onto the flight deck of the aircraft carrier the USS Ronald Reagan in the Philippine Sea in October, injuring 12 personnel on board.