Maribor (Slovenia) (AFP) - Slovenia on Thursday started burying the remains of some 800 alleged Nazi collaborators killed by communist forces in the aftermath of World War II, and discovered in a giant mine in 2009.

"It is our human and state duty to bury in a civilised way the victims of war and post-war killings," Slovenian President Borut Pahor said at the burial ceremony in the northern city of Maribor.

"We're not changing history, we are changing our future. This represents a step forward," he added.

Croatian Prime Minister Andrej Plenkovic also attended the ceremony, the first in a series of state burials expected to continue until the end of 2017.

The bodies were discovered in March 2009 in the Huda Jama mine, some 80 kilometres (50 miles) northeast of the capital Ljubljana.

Experts say most of the victims were Croats and Slovenes who had been executed for collaborating with the Nazis, often without trial.

Authorities believe the mass grave may have contained up 2,500 bodies.

Croatia's President Kolinda Grabar Kitarovic, who visited Maribor earlier on Thursday, said "the historic truth has to be determined to ensure a better future".

A special government commission has so far registered 700 possible mass grave sites containing up to 15,000 bodies in Slovenia, a former Yugoslav republic until 1991 when it declared independence.