An Allied bomb left over from World War II has exploded in Germany, killing three military engineers who were trying to defuse it.

The blast occurred in the central city of Goettingen on Tuesday after construction workers building a sports stadium discovered it in a densely populated area.

Bomb disposal experts were called to the scene to defuse the 500 kilogramme device, which police said was likely to be British. But it exploded before they could neutralise the device.

Another six members of the bomb disposal team were injured in the blast, but all were expected to survive.

"We are deeply shocked by the horrible events last night," Uwe Schuenemann, the Lower Saxony state interior minister, said.

"We are mourning in deep sympathy with the families of the three men."

Explosion without warning

Schuenemann said the men who were killed were all experienced bomb disposal experts, who had defused hundreds of devices in the past.

They were called to the scene after thousands of residents were evacuated from the area. But before they could begin work, the bomb exploded without warning.

Bombs left over from the second world war are regularly found in Germany. Between 400 and 500 people are employed nationwide in removing them, and experts expect unexploded ordnance to pose problems for decades to come.   

"After the war, there was a building boom, and buildings often went up in areas where there were bombs," Volker Scherff, the head of the association of German Explosive Ordnance, said.

"Those bombs are still there and when construction work is done today, the ground must always be actively searched for ordnance."

Last month, 9,000 residents of the Berlin district of Zehlendorf were evacuated when a 500-kilogramme bomb was unearthed. That discovery followed the closure of Berlin's main airport in April after a British bomb left over from the war was found nearby.

Berlin authorities believe there could be up to 3,000 bombs still buried in the city.