TOKYO (AP) — A major airport in northern Japan was closed Tuesday after construction workers found an unexploded bomb believed to be from World War II.

All 92 flights in and out of Sendai airport were cancelled after the 250-kilogram (550-pound) bomb was uncovered during construction near a runway, local police official Hiroshi Ouchi said. The bomb was identified as American-made and is believed to be a dud from World War II.

It appeared to have a working detonator, and a military bomb squad was considering whether to move the bomb or explode it on the spot. Sandbags were piled up around the bomb, and local officials said evacuations of nearby homes were being considered. They added that the area had been sealed off and there was no immediate concern the bomb would explode.

The Sendai airport, a regional hub for travel into northern Japan, was closed for months due to severe damage from last year's tsunami. It is still undergoing reconstruction.

Officials said the airport would reopen as soon as the bomb was considered secure, possibly by Wednesday morning. But they said it could take a week to actually dispose of it or erect barriers to prepare the site for a controlled detonation.

The United States heavily bombed Japanese cities during World War II, and finding unexploded bombs is not unusual, even 67 years after Japan's surrender. Dozens of duds are uncovered in Tokyo each year, and even more are found on the southern island of Okinawa, which was the site of the most intense fighting during the war.

Many of them are found at construction sites. The rusty condition of the bombs can make them prone to detonate when moved, but injuries are rare. Experts say it could take several decades to remove all of the unexploded ordnance.

Last week, hundreds of residents in central Tokyo were evacuated so that bomb squads could remove a 220-kilogram dud buried there.