Search-and-rescue operations for the remaining five continue near the crash site, about 100 kilometres off the cape of Muroto in southwestern Japan.

The incident adds to a growing list of US 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 per cent from fiscal years 2013 to 2017. At least 133 service members were killed in those incidents, it said.

Congressional leaders have called the rash of accidents a “crisis” and blamed it on continuous combat operations, deferred modernisation, lack of training and ageing equipment.

US 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 US presence in the country. A series of emergency landings and parts falling from US military aircraft have highlighted safety concerns.

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

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In November, a US Navy fighter from the USS Ronald Reagan crashed off Okinawa, while a MN-60 Seahawk helicopter from the same aircraft carrier crashed in the Philippine Sea shortly after take-off, leaving a dozen sailors with injuries.

In February, the mayor of a small town in Hokkaido in the far north of Japan, demanded that aircraft at a nearby US airbase be grounded after a an F-16 Fighting Falcon jettisoned its external fuel tanks into a lake.

The US Navy has also had a number of incidents. In June 2017, the USS Fitzgerald collided with a container ship some 60 nautical miles off the destroyer’s home port in Japan. of Yokosuka. Seven of the warship’s crew were killed and two senior officers were relieved of their duties.

Two months later, the USS John S. McCain was involved in a collision with a merchant ship off Singapore that left 10 of her crew dead and five injured.

US Ambassador William Hagerty thanked Japan’s military for their search-and-rescue efforts and confirmed the incident occurred during a refuelling exercise.

“My heart goes out to the families and colleagues of Marines involved in this tragedy,” Hagerty said at an event at Waseda University in Tokyo. “They risk their lives every day to protect Japan and to protect this region and sometimes they pay the greatest costs. So I want to emphasise this security alliance that we have is critical and it is moving forward to the right direction.”

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The Marine Corps said in a statement that the incident occurred at around 2am local time, about 320km off the Japanese coast.

The two aircraft had launched from Marine Corps Air Station Iwakuni and were conducting regular training when the incident occurred, it said.

Despite this being the latest in a string of accidents, Jun Okumura, a political analyst with the Meiji Institute of Global Affairs, said it was unlikely to have a “direct affect on the bilateral relationship” between the governments of Japan and the US.

“This exercise was being conducted out to sea and away from any civilian interests, so there was very little danger to anyone, but it will again feed into the general narrative that these exercises need to be safe,” he said.

“Obviously the opposition will try to make a big deal of the incident in the Diet, but I would say this is more an issue for the US military and not something that is going to push local people over the threshold in terms of opposition to the US forces here.”

Additional reporting by Julian Ryall