Police have cordoned off a zone around the site where a World War Two bomb was found on Tuesday in the German city of Dresden after initial attempts to defuse it failed.
There was a small explosion and fire on Wednesday as experts tried to make the bomb safe.
They will not approach the bomb again until the fire has been extinguished and the site has cooled down.
Allied bombers devastated the eastern city in the war.
Police have urged people to stay well away from the zone amid fears of a major explosion.
Many flights to and from Dresden airport have been cancelled because of the risk that the bomb could explode.
About 9,000 people who were living near the site have spent two nights in emergency accommodation. They include many elderly people evacuated from two care homes.
The bomb was found during construction work at the intersection of two streets – Löbtauer and Werner – in central Dresden.
In February 1945 an estimated 25,000 people died in the British and American bombing of Dresden. It created a firestorm that left 33 sq km (12 sq miles) of the city in ruins.
The intense Allied air campaign against Nazi Germany left a legacy of thousands of unexploded bombs. Builders regularly unearth such bombs in German cities.
The northern city of Kiel is preparing to make a bomb safe on Friday – an operation that will involve evacuating about 4,300 residents.
Last month about 10,000 people were evacuated in central Berlin during a successful operation to defuse a British wartime bomb.