Two Japanese boys have brought a World War II bomb to class, flustering teachers on the island of Okinawa, which was the scene of the Pacific conflict's bloodiest battle.

The two 12-year-old boys said they found the bomb last week in a yard near the school in Okinawa, where residents still unearth hundreds of dud explosives six decades after the war.

"We immediately realised the blasting fuse had come off the bombshell," said Yoshiyasu Henzan, vice principal of Shonan elementary school.

"We reported it to police through the local education board.

"The children apparently thought the bomb would be good study material at school," he said, adding that the school has given a special class in peace studies for students.

The US-made bombshell of about 30 centimetres in length was swiftly taken away by the Japanese bomb-disposal squad, the teacher said.

Okinawa was the bloodiest battle of the Pacific war, with US forces unleashing an 83-day air and amphibious assault dubbed the "Typhoon of Steel" by locals.

Some 190,000 Japanese died, half of them Okinawan civilians.

Earlier this month, an Okinawan resident working on an underground water pipe was severely injured in the face when another World War II bomb went off.

The bomb-disposal squad of the Japanese military unearthed 445 bombs in Okinawa in just two weeks after the explosion, the local Ryukyu Shimpo daily reported.

-AFP