All eight service members aboard the U.S. military Osprey CV-22 aircraft that crashed into waters off southwestern Japan are believed dead, the Pentagon and White House said Tuesday, making it the aircraft’s deadliest-ever accident.

The crash — the first fatal accident involving an Osprey in Japan — has rekindled concern over the aircraft’s safety, with Japan’s defense chief saying that concerns have yet to be allayed despite a halt in operations of the crashed aircraft’s unit.

U.S. President Joe Biden said Tuesday in Washington that he and his wife were “heartbroken” over the loss of the service members, and that the search-and-rescue operation had ended “after an extensive and weeklong effort” following the Nov. 29 crash in waters off Kagoshima Prefecture’s Yakushima Island.

“Our service members and their families are the backbone of our nation. We owe them everything. Jill and I are praying for the families and friends who lost a loved one in this terrible accident,” Biden said in a statement, thanking Japan for its assistance in the hunt for the Osprey and the ongoing recovery effort.

“Our entire nation mourns this tragic loss,” he added.

Earlier, the U.S. Air Force Special Operations Command said that the transition from rescue to recovery operation meant “that survivors are unlikely,” and that the recovery would “now focus on locating and recovering all remains and aircraft debris.”

It said that, of the eight crew members — who were all in their 20s and 30s — the remains of three had been recovered, while those of another three had been located and were in the process of being recovered. The remains of two other crew members have yet to be found.

“Personnel and assets across five military branches alongside our Japanese allies supported the search and rescue operations and will continue to support recovery operations,” the command added.

In a letter to Biden, Prime Minister Fumio Kishida said Wednesday that he was “deeply saddened” by the tragedy and expressed his condolences.　

“Prime Minister Kishida expressed his sincere gratitude to U.S. military personnel in Japan, who are away from their homes and families, working day and night in distant lands to maintain the peace and security of Japan and the surrounding region,” he said, according to the Foreign Ministry.

Japan has grounded the Self-Defense Forces' own fleet of 14 Ospreys and initially asked the United States not to conduct flights of the aircraft except for ongoing search operations. But the Defense Ministry later said that U.S. military flights involving Osprey variants other than the CV-22 type that crashed were continuing after undergoing safety checks.

It was not immediately clear how long the halt of CV–22 Osprey operations would continue.

"Concerns have not yet been allayed, so we need to continue to receive information" on the crash from the U.S. side, Defense Minister Minoru Kihara said Tuesday.

While the Osprey has a history of accidents, including a 2016 crash off Nago, Okinawa Prefecture, last week's crash is its deadliest since the aircraft officially entered into operational service in 2007 — though an April 2000 crash in Arizona during testing killed all 19 U.S. Marines on board.

Yakushima is part of the Nansei Island chain, where both the U.S. and Japan have looked to bolster their presence amid growing military assertiveness by China. However, the accident has highlighted anxiety among the public about the deployment of Ospreys in the area and across Japan, while also stoking fears it could dent the U.S.-Japan alliance.

Asked Monday about the U.S. response to the crash, Pentagon spokesperson Sabrina Singh said Washington was “taking all appropriate measures” and had “already started sharing information about the accident with our Japanese partners and ... should continue to do so in a timely and transparent manner.”

U.S. Defense Secretary Lloyd Austin said in a statement Tuesday that the American military was continuing to gather information on the crash, and “will conduct a rigorous and thorough investigation.”

The main part of the Osprey CV-22 was found in an area about 2 to 3 kilometers south-southwest of where it disappeared from radar, in the opposite direction from Yakushima Airport, where it was aiming to make an emergency landing, the Asahi daily reported, with the pilots apparently having lost control of the aircraft.

The U.S. Air Force CV-22 was one of six assigned to Yokota Air Base in the western suburbs of Tokyo headed for the Kadena Air Base in Okinawa Prefecture from another U.S. base in Iwakuni, Yamaguchi Prefecture, when it went down.

Media reports have said it is highly likely that a fire broke out in an engine of the CV-22 while it was flying in helicopter mode with the rotor facing up, with flames seen in its left engine, and information that the aircraft was spinning before it crashed.

Ospreys have an engine structure integrated with a rotor at the end of the left and right main wings, and the rotor angle can be adjusted. When the rotors are directed upward, vertical takeoffs and landings are possible. High-speed flights, meanwhile, are possible when the rotors are tilted forward.