The bodies of six soldiers, believed to be Japanese troops who fought in World War II, have been discovered in a reopened cave in the tiny Pacific nation of Palau.

The site — one of around 200 sealed caves on the island of Pelileu — was recently opened again for the first time in nearly 70 years.

The caves were used when US and Japanese forces fought a fierce battle on the island's beaches in September 1944.

Steve Ballinger, operations director with non-government organisation Cleared Ground Demining, said the bodies of the six soldiers would be repatriated.

"The cave itself is in an area known as the promontory which is the defining point on the west coast of the island of Pelileu," he told Radio Australia's Pacific Beat program.

"At that location was an anti-tank gun in a heavily fortified concrete bunker and it took a number of days actually to capture this fortified position.

"It's my understanding that those [bodies] were the crew, perhaps the officer and his men that were manning that gun ... a number of US soldiers died in that vicinity as well."

Mr Ballinger said other war relics were also found in and around the cave.

The team exploring the Palau cave where the remains of six bodies were found is briefed. ( Supplied: Steve Ballinger )

"During the detection and investigation we located hand grenades, large projectiles, small arms ammunition and (an) array of explosive remnants of war," he said.

The decision to open the caves in Palau comes ahead of an imperial visit next week by Japanese emperor Akihito and empress Michiko.

One of the caves sealed off after a fierce World War Two battle between US and Japanese forces in 1944. ( Supplied: Richard W. Brooks )

The team tasked with making the caves safe for anthropologists to investigate has been operating in Palau to clear remnants of WWII ordnance for six years.

It is made up of 18 Palauans, two Britons and one person each from South Africa and Australia.

Mr Ballinger said the team had been concerned about possible booby traps when they first entered the cave.

"[We] had to check in the floor and the entrance of the cave to allow Japanese and US anthropologists and archaeologists in there," he said.

"It was very tight, it was a very difficult entrance to the cave, a very harsh environment to work in."

He said the cave the remains were found has since been resealed.

"A lot of field caves will contain considerable quantities of human remains and as such you could designate them as graves and wouldn't be appropriate to use as tourist attractions," he said.

An estimated 10,000 Japanese soldiers died during the fighting in the country, and the remains of some 2,600 troops have never been found.