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.

Five U.S. Marines were missing after two Marine Corps aircraft collided in mid-air and crashed into the sea off the coast of Japan on Thursday, in what U.S. officials said may have been a refueling exercise gone wrong.

Japan's defense ministry said its maritime forces had so far found two of the seven Marines who were aboard the aircraft — an F/A-18 Hornet fighter jet and KC-130 Hercules — at the time of the incident.

One was in a stable condition at Marine Corps Air Station Iwakuni, while the second had been found about 10 hours after the collision and brought aboard a Japanese military vessel, the ministry said. No other details about the second Marine were known, a ministry spokesman said.

Search and rescue efforts for the remaining five continued.

The incident adds to a growing list of U.S. military aviation accidents around the world in recent years, prompting hearings in Congress to address the rise.

The Military Times reported earlier this year that aviation accidents jumped nearly 40 percent from fiscal years 2013 to 2017. At least 133 service members were killed in those incidents, it said.

U.S. military accidents are a sensitive topic in Japan, particularly for residents of the southern prefecture of Okinawa, which is 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 highlighted safety concerns.

"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."

The Marine Corps said in a statement the incident occurred around 2 a.m. local time in Japan (1700 GMT Wednesday) about 200 miles off the Japanese coast.

The two aircraft had launched from Marine Corps Air Station Iwakuni and were conducting regular training when there was a "mishap," the Marine Corps said.

The Marine Corps did not elaborate on the nature of the incident. A U.S. official, speaking on condition of anonymity, said it occurred during a refueling exercise.

Officials who spoke to Reuters on condition of anonymity were unsure precisely how the mishap occurred but none suspected foul play. An investigation has begun.