A massive World War II bomb exploded during a delicate operation on Tuesday to defuse the five-ton device in a channel near the Baltic Sea but no one was harmed, Polish officials said.

The device - nicknamed "Tallboy" and also known as an "earthquake bomb" - was dropped by the Royal Air Force in an attack on a Nazi warship in 1945.

It was discovered in 2019 embedded at a depth of 12 meters with just its nose sticking out during dredging close to the port city of Swinoujscie in northwest Poland.

More than six meters long, it was laden with 2.4 tons of explosives - equivalent to around 3.6 tons of TNT.

The navy had said earlier it had ruled out the traditional option of a controlled explosion for fear of destroying a bridge located some 500 meters away.

Instead, it had planned to use a technique known as deflagration to burn the explosive charge without causing a detonation, using a remotely controlled device to pierce through the shell to begin combustion.

But in the end "the deflagration process turned into a detonation," said Grzegorz Lewandowski, spokesman for the Polish navy's 8th Coastal Defence Flotilla based in Swinoujscie.

"There had been no risk to the individuals directly involved," he said.

A Swinoujscie city hall spokesman told AFP he had not heard any reports of anyone injured during the operation by military divers, nor of any damage done to the city's infrastructure. 

Before the operation began this week, Lewandowski had called it "a very delicate job," adding that "the tiniest vibration could detonate the bomb."

Around 750 local residents had been urged beforehand to evacuate from an area of 2.5 kilometers around the bomb.

Halina Paszkowska had said the "main danger" for her was the risk of catching COVID-19 in a sports hall where residents were given shelter during the operation.

AFP