Three experts working to defuse a bomb from the second world war were killed when the device exploded, injuring six others, police said today.

Some 7,000 residents from the central German town of Göttingen, where the 500kg bomb was found, were still being evacuated when it blew up late on Tuesday.

Construction workers had found the device about six metres below the ground where the city is building a sports arena.

The three dead men, aged 38 to 55, were experienced in defusing bombs and it was not clear why it exploded, Göttingen police president Robert Kruse said at a press conference today. He said two other experts were severely injured and four others were treated for shock..

City spokesman Detlef Johannson said the team was still preparing to remove the detonator when the bomb exploded.

Unexploded bombs from Allied bombardments and the first world war are found regularly in Germany. Only a few days before, another bomb found in Göttingen was successfully defused.

Every German state has dozens of specialists trained to defuse old bombs, and accidents are rare. Hundreds of police and firefighters are usually involved in helping evacuate people before experts attempt to make safe a bomb.