A huge unexploded World War II bomb has been found buried near the runway of one of Japan's busiest regional airports, forcing all flights to be cancelled Tuesday, officials said.

A worker rebuilding drainage systems near Sendai Airport -- which was swamped by last year's tsunami -- uncovered the 500-pound (225-kilogramme) bomb, believed to have been dropped by US forces, on Monday evening.

All 92 domestic and international flights scheduled for Tuesday were cancelled as bomb disposal experts from the Japanese military worked on making the device safe.

Troops were building a three-metre-high (10-foot-high) wall of sandbags around the bomb and would deal with it on Wednesday morning after explosives experts had checked its fuse, an airport official said.

The official said said the bomb was 110 centimetres (43 inches) long and 35 centimetres in diameter.

If the bomb explodes, fragments could be scattered more than a kilometre (more than 1,000 yards), NHK said, adding that local officials were considering an evacuation.

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.

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.