Image copyright Reuters Image caption One of the two planes involved in the incident was a KC-130 refuelling tanker (like the one seen in this archive picture)

Five missing Marines have been declared dead by the US military, nearly a week after two aircraft collided and crashed into the sea off Japan.

A massive search and rescue operation was mounted after the 6 December incident, involving a F/A-18 fighter and a KC-130 refuelling tanker carrying seven crew in total.

One crew member survived, and Tuesday's announcement brings the final death toll to six

The exact cause remains unclear.

The crash is said to have occurred during a refuelling exercise but the US military has not confirmed this.

"Every possible effort was made to recover our crew and I hope the families of these selfless Americans will find comfort in the incredible efforts made by US, Japanese, and Australian forces during the search," said US Marine Corps Lt Gen Eric Smith.

The identities of the five Marines declared dead have not yet been released.

The incident occurred in the early hours of 6 December about 200 miles (320km) off the coast of south-western Japan.

The US planes had taken off from Marine Corps Air Station Iwakuni near Hiroshima and "were conducting regularly scheduled training when the mishap occurred", the III Marine Expeditionary Force said.

Two Marines were rescued after the incident but one was declared dead. He has been identified as Capt Jahmar Resilard, the pilot of the F/A-18.

There were five personnel on the C-130 and two on the F-18.

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

There have been several recent incidents involving US military aircraft. 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.