U.S. President Joe Biden said Tuesday he and his wife are "heartbroken" over the loss of eight American service members after a military Osprey aircraft crashed off the coast of southwestern Japan last week during a routine exercise.

"Since then, we were focused on search and rescue, which ended today after an extensive week-long effort," Biden said in a statement, praising U.S. military personnel and their families as "the backbone" of the country.

"Our entire nation mourns this tragic loss," Biden said. He also thanked the Japanese government for its cooperation in carrying out the search and rescue operations.

As of Monday, six of the eight U.S. service members on board had been confirmed dead, with the remains of three recovered. The U.S. military is still trying to locate the bodies of two of the missing.

An object believed to be debris from a crashed U.S. military CV-22 Osprey aircraft is unloaded at a port on the island of Yakushima in Kagoshima Prefecture on Dec. 4, 2023. (Kyodo)

The Air Force Special Operations Command separately said Tuesday that all eight have been identified and the U.S. military's search and rescue operations have switched to search and recovery mode.

"The transition from a rescue operation to a recovery operation occurs when the determination is made that survivors are unlikely," the command said, adding that the military's efforts will focus on locating and recovering all remains and aircraft debris.

It said the eight, including pilots and flight engineers, were all in their 20s or 30s.

The U.S. Air Force's CV-22 tilt-rotor aircraft, assigned to the 353rd Special Operations Wing at Yokota Air Base in the western suburbs of Tokyo, disappeared from radar at around 2:40 p.m. last Wednesday off Yakushima Island and crashed into nearby waters.

The cause of the accident -- the deadliest involving a U.S. Osprey since the aircraft's combat debut in 2007 -- is under investigation.

Ospreys have a history of mishaps, including fatal crashes. In late August, a U.S. Marine Corps Osprey crashed on a remote Australian island during a joint exercise with countries including the Philippines, killing three of the 23 who were aboard the aircraft.

Last week's crash has rekindled concerns about the safety of Ospreys in Japan, a key U.S. security ally.

Japan has grounded its forces' own fleet of 14 Ospreys and asked the United States not to conduct Osprey flights except for its ongoing search operations.

But U.S. military flights involving Osprey variants other than the one that crashed have not been completely suspended.

U.S. Defense Secretary Lloyd Austin also released a statement on Tuesday, saying, "We continue to gather information on this tragic incident, and we will conduct a rigorous and thorough investigation."

Austin touched on those killed in the crash and offered condolences to their families. He added that the United States is grateful to Japan's Coast Guard, Self-Defense Forces and local communities, including fishermen, for assisting the search, but made no reference to Tokyo's requested suspension of Osprey flights.

A Japan Maritime Self-Defense Force ship searches for a U.S. military Osprey aircraft off Yakushima Island in the southwestern Japan prefecture of Kagoshima on Dec. 1, 2023, after it crashed two days earlier. (Kyodo) ==Kyodo

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