The remains of at least 8,000 victims of the Nazis have been found in a mass grave near the site of the Soldau concentration camp in northern Poland.

The bodies had been buried but were then dug up and burnt in an attempt by Nazi officials to hide evidence of their crimes. 

Ashes weighing 17.5 tonnes were unearthed last month in two pits each 10 feet deep outside the walls of Soldau last month.

The graves had been discovered by Poland's Institute of National Remembrance (IPN). 

On Wednesday, a stone monument was unveiled at the site with an inscription reading: “Unknown martyrs fallen for being Polish. 1939-44.”

IPN President Karol Nawrocki said at the ceremony, “The Germans decided to avoid the responsibility for the crimes they had committed."

He added that "the unburned remains" had been hidden in the ground "so that the crime would not see the light of day and no one could be held responsible”.

“The [Nazi] cover-up has failed because the IPN is determined to search for the victims and heroes of WWII and will never allow even one of them to be forgotten.”

Soldau was set up in 1939 in the town of Działdowo after the Germans invaded Poliand.

The camp housed around 30,000 inmates, 13,000 of whom were murdered.

The ashes found last month are the burnt remains of bodies dug up in 1944 by Jewish concentration camp inmates forced to act as slave labourers.