German President Frank-Walter Steinmeier asked Greece for forgiveness for crimes committed during the Nazi occupation, in comments at a former concentration camp near Athens on Thursday.

"Unimaginable atrocities were committed under German auspices in the Haidari camp," Steinmeier said at the camp, where up to 25,000 people were interned during occupation. "We bow before the victims."

During WWII, between 60,000 and 70,000 Jewish Greeks were murdered in mass shootings and other atrocities. Steinmeier said Germany's moral and political responsibility for this must not be forgotten.

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Steinmeier's visit, on the anniversary of the liberation of Athens, came after Greek Prime Minister Alexis Tsipras's government renewed Greece's oft-repeated claims that Germany still owes vast sums in war reparations.

Greek officials said last month Athens will seek parliamentary approval this year to further their claims against Germany, which date back to the 1990s. A 2016 parliamentary report found Germany owed more than €270 billion ($312 billion), including the repayment of a €10 billion "loan" the Bank of Greece was forced to grant the Nazis.

Berlin says the issue was definitively resolved in a previous, wider post-war agreement.

Read more: Nazis' stolen 'loan' from Greek bank: Will Germany pay it back?

Calls for European unity

Speaking on European unity, Steinmeier said he shared Tsipras's concerns about the future of Europe, given the growing threat of populism. He said European citizens needed to be convinced that crises can be overcome together, while extreme and populist positions must be repressed.

Specifically on the issue of migration, he noted that Greece was on the front line and that consensus on migration policy "is not possible without solidarity."

He called for a new chapter in Greece-Germany relations, after the end of EU bailouts and the associated eight years of unpopular austerity.

Read more: France to Germany: Stop right-wing populism by making decisions

aw/ng (dpa, AFP, AP)