Japanese officials have expressed concern that the US military is continuing to fly Osprey aircraft in the country without providing adequate information about a fatal crash this week despite repeated requests.
One crew member was killed and seven others are missing, along with the aircraft.
The cause of Wednesday’s crash, which occurred during a training mission, is still under investigation.
Search operations widened on Friday with additional US military personnel joining the effort while Japanese coast guard and military ships focused on an undersea search using sonar.
The Pentagon said on Thursday that US Ospreys continue to operate in Japan, and deputy press secretary Sabrina Singh said she was not aware of an official request from Japan to ground them.
“We are concerned about the continuing Osprey flights despite our repeated requests and the absence of a sufficient explanation about their safety” from the US military, chief cabinet secretary Hirokazu Matsuno said on Friday.
The US-made Osprey is a hybrid aircraft that takes off and lands like a helicopter but can rotate its propellers forward and cruise much faster, like a plane, during flight.
Ospreys have had a number of crashes, including in Japan, where they are used at US and Japanese military bases, and the latest crash rekindled safety concerns.
Japanese officials say they asked the US military to halt Osprey flights in Japan except for those involved in the search operations.
Defence minister Minoru Kihara said he met the commander of US Forces Japan, Ricky Rupp, on Thursday afternoon and repeated his request that flights be allowed only after the aircraft’s safety is confirmed.
He acknowledged that he did not specifically use the words “grounding” or “suspension”.
Mr Kihara said he asked Mr Rupp to explain what measures are being taken for Osprey flights in Japan in response to the crash.
On Thursday, Japanese foreign minister Yoko Kamikawa met US ambassador to Japan Rahm Emanuel and asked the United States “to promptly provide information to the Japanese side”.
US Air Force Special Operations Command said the CV-22B Osprey that crashed was one of six deployed at Yokota Air Base, home to US Forces Japan and the Fifth Air Force, and was assigned to the 353rd Special Operations Wing.
The aircraft had departed from the US Marine Corps Air Station Iwakuni in Yamaguchi prefecture and crashed on its way to Kadena Air Base on Okinawa, Japanese officials said.
A total of 44 Ospreys have been deployed at US and Japanese military bases in Japan. In Okinawa, where about half of the 50,000 American troops in Japan are based, Governor Denny Tamaki called on Japan’s defence and foreign ministries to request the US military to suspend all Osprey flights in Japan, including in search operations.
“It is extremely regrettable that Ospreys are still flying in Okinawa,” Mr Tamaki said in a statement on Thursday. “I have serious doubts about Osprey safety even for their search and rescue operations.”
