WASHINGTON (Reuters) - One U.S. Marine was killed and rescue teams were searching for five others missing after two Marine Corps aircraft collided in mid-air and crashed into the sea off the coast of Japan during a refueling exercise, officials said on Thursday.

A Japan Coast Guard patrol vessel and U.S. Navy airplane conduct search and rescue operation at the area where two U.S. Marine Corps aircraft have been involved in a mishap in the skies, off the coast of Kochi prefecture, Japan, in this aerial view photo taken by Kyodo December 6, 2018. Mandatory credit Kyodo/via REUTERS

Japanese and American officials said they had so far found two of the seven Marines who had been aboard the aircraft, an F/A-18 Hornet fighter jet and a KC-130 Hercules.

“One of the recovered Marines is in fair condition and the other has been declared deceased by competent medical personnel,” a U.S. Marine Corps statement said.

The deadly crash is the latest in a string of U.S. military aviation accidents around the world in recent years, which have prompted hearings and criticism in Congress over aircraft safety.

U.S. President Donald Trump expressed gratitude to Japan for its quick assistance with search and rescue efforts.

“My thoughts and prayers are with the @USMC (U.S. Marine Corps) crew members who were involved in a mid-air collision off the coast of Japan,” he tweeted.

The incident occurred around 2 a.m. local time in Japan (1700 GMT Wednesday) about 320 km (200 miles) off the Japanese coast. The Marine Corps said the aircraft were conducting routine training at the time, including refueling training.

The U.S. military did not speculate on the cause of the collision.

“The incident is regrettable, but our focus at the moment is on search and rescue,” Japanese Defense Minister Takeshi Iwaya told a news conference. “Japan will respond appropriately once the details of the incident are uncovered.”

A U.S. Navy P-8A patrol and surveillance aircraft was helping in search and rescue efforts along with Japanese authorities, the Marine Corps said, adding that the incident was under investigation.

QUESTIONS ABOUT READINESS

The incident raises questions about military readiness, something U.S. Defense Secretary Jim Mattis has said is a priority for him.

Congressional leaders have called the rash of accidents a crisis and blamed it on continuous combat operations, deferred modernization, lack of training, and aging equipment.

Last year, 16 service members were killed after a military transport plane crash in rural Mississippi.

A military investigation said the primary cause of that crash was a propeller blade that went into the aircraft’s fuselage. The investigation added that the propeller had not received proper maintenance and corrosion had been overlooked.

In August 2017, the U.S. Marine Corps ordered its aircraft squadrons to suspend flight operations for 24 hours to review safety of aircraft.

U.S. military accidents are a sensitive topic in Japan, particularly for residents of the southern prefecture of Okinawa, home to the bulk of the U.S. presence in the country. A series of emergency landings and parts falling from U.S. military aircraft have sparked concerns over safety.