Katsutoshi Kawano, as saying that search teams had spotted debris floating in the sea.

How did the events unfold?

A Facebook posting by the III Marine Expeditionary Force said the incident took place 200 miles (320km) off the coast.

The US planes had taken off from Marine Corps Air Station Iwakuni and "were conducting regularly scheduled training when the mishap occurred".

Image copyright AFP Image caption An F/A 18E Super Hornet on the USS Ronald Reagan. It is not yet known which variant was involved in the incident

The BBC's Rupert Wingfield-Hayes in Tokyo says that air-to-air refuelling is a difficult and potentially dangerous flight operation, especially when done at night.

He says it is not clear what the weather conditions were like but overnight there was widespread cloud and rain across the Japanesearchipelago.

The KC-130 is an extended-range tanker version of the C-130 and is used for mid-air refuelling.

The McDonnell Douglas F/A-18 Hornet is a fighter and attack aircraft and can carry a wide range of missiles and bombs.

The US has more than 50,000 troops stationed in Japan, more than 18,000 of them in the US Marine Corps.

The US has had problems with reliability of its aircraft in Japan. In November an F/A-18 Hornet crashed into the sea south of Okinawa. The two pilots ejected and were rescued.

Last December, part of a US helicopter crashed on to a school in Okinawa, renewing tensions with the local population.

Over the past years, a number of accidents and crimes have led to growing local opposition to the US base there.