An unexploded World War II bomb found buried near the runway of a busy Japanese airport was safely defused.

Military experts spent an hour removing the fuse from the 500-pound (225-kilogramme) bomb, believed to have been dropped decades ago by US forces, on what is now Sendai aiport in the country's northeast.

The bomb was found last month by workers building drainage systems at the airport, which was swamped by last year's tsunami.

All flights were cancelled immediately after the discovery as troops built a six-metre- (20-foot-) high protective casing around the device, using concrete blocks and sandbags to protect aircraft as they devised a way to get rid of it.

Normal operations, including domestic and international flights, resumed with aircraft taxiing within sight of the defensive structure.

The operation on Wednesday morning took less than an hour, with the bomb being sent to a military base in Yamagata prefecture to be destroyed, said a spokesman for the Ground Self Defence Force, Japan's army.

Nearly 70 years after the war ended, unexploded bombs and shells are still occasionally found in Japan, particularly on the southern island of Okinawa, the site of an extremely bloody battle towards the end of World War II.

Less than a month ago, Japanese troops defused a similar 500-pound WWII bomb found in a busy business district of Tokyo.

Sendai Airport was devastated in the quake-triggered tsunami that hit Japan in March 2011. Footage posted on YouTube of waves covering the runway has been seen by more than 20 million people.