BERLIN, Germany (AFP) — Bomb disposal experts defused an unexploded American World War II shell in the western German city of Koblenz on Saturday, as a massive evacuation began in neighbouring Frankfurt to clear an even bigger bomb.

Around 21,000 people had to leave the area in Koblenz as the 500-kilogramme (1,100 pound) bomb was successfully defused, before the fire brigade allowed residents to return to their homes.

The operation came a day before three times as many people must leave their homes in Frankfurt to allow for a Royal Air Force bomb to be neutralised, in the biggest evacuation of its kind in post-war Germany.

The British bomb found in central Frankfurt weighs 1.8 tons and its disposal, planned for Sunday, has prompted a much larger evacuation protocol, affecting more than 60,000 residents.

The city said police will enforce a 1,500-metre radius evacuation zone starting early Sunday and residents will probably need to stay away until 8:00 pm.

Seven decades after the end of the Second World War, unexploded bombs from intense Allied raids on Nazi Germany are still occasionally found in building sites, forests and fields and sometimes even in private gardens.