The row between Germany and Greece over war reparations has intensified after Athens hit back at Berlin’s description of its demand for a staggering €278.7bn (£202bn) in compensation as “stupid”.

Insisting that Greece’s leftist-led administration had “a historical duty” to seek compensation for atrocities committed by Nazi forces between 1941-44, the politician in charge of the campaign said on Wednesday that he welcomed the German reaction.

“The response may have been ‘this is foolish, you have plucked this number out of the blue’ but for me it was also very positive,” Costas Isychos, the deputy defence minister, told the Guardian. “There was an admission that despite disagreeing with the figure a debt is owed, and that is very good.”

On Tuesday, Germany’s economy minister and vice-chancellor, Sigmar Gabriel, not only branded the demand boneheaded, but suggested it had been motivated by Athens’ interest in squeezing a bit of leeway out of its eurozone partners to overcome its debt crisis.

“And this leeway has nothing to do with the second world war or reparations,” said Gabriel, who leads the Social Democrats, the junior partner in the ruling coalition of the chancellor, Angela Merkel.

Berlin has provided the bulk of the €240bn bailout that has kept the insolvent Greek economy afloat since 2010.

But while the claim exceeds that amount, Isychos vehemently denied it was linked to the country’s economic plight, or the tough austerity measures Berlin has pressed for in exchange for international aid. He implied the figure could in fact be much bigger when interest payments were also taken into account.

“We will push for this as much as we can,” he said, describing the war reparations as “an open wound” for a country that had suffered one of the most brutal occupations under Hitler.

“We have to close this wound. It is not related, whatsoever, to Greek debt or any policies connected to the memoranda,” he said, referring to the deeply unpopular bailout accords. “They are two very different issues. The left has always been sensitive to this issue.”

The timing of the demand was coincidental, he said. “It is a very good coincidence for the Greek people but a very bad coincidence for those who want to connect it to the memoranda.”

The Greek prime minister, Alexis Tsipras, has accused Berlin of deliberately employing “legal tricks and delay” to avoid what he described as a moral obligation to compensate war crimes. Meeting Merkel in Berlin two weeks ago, he pressed the point again. The German leader appeared to open the door to discussion, but it was quickly shut by Berlin’s finance ministry.

The German government has repeatedly described the issue as closed, saying the country honoured its obligations, including a 115m deutschemark payment made to Greece in 1960 (worth about £150m today).

“For me the figure of €278.7bn of supposed war debts is neither comprehensible nor sound,” Eckhardt Rehberg, a budget expert for Merkel’s conservative party, was quoted as saying this week. “The issue of reparations has, for us, been dealt with both from a political and a legal perspective.”

But Isychos, who was born in Buenos Aires and raised in Canada, said Athens had “very detailed” documentation to back its claim. Greek officials had 400,000 pages of records obtained from the US national archives chronicling atrocities committed by the Third Reich.

Crimes ranged from reprisal executions to the pillaging of the country’s cultural heritage and an interest-free forced loan, officially estimated by the general accountancy office at €10.3bn, which was extracted from the Bank of Greece to fund Hitler’s Africa campaign. The Greek defence ministry is in the process of translating the data and digitalising microfilms.

“The occupation forces were extremely methodical in their reports to superiors, listing massacres and the shooting of victims, including women and children, the destruction of homes, you name it,” he said. “Greece, for example, was the biggest exporter to Nazi Germany of precious metals such as chrome. Some 279,000 tonnes were exported but never paid for.”

Soon, experts would also be scouring historical archives obtained from Russia, he said. “I formally requested the archives two weeks ago when I visited Moscow and was told that they do indeed have them,” he said of records that ended up in the possession of Russian and American forces at the end of the war.

Athens’ demand for repayment of its forced wartime loan has found support from the German opposition, with members of the Greens and the far-left Die Linke party saying Berlin should honour the debt. Syriza has strong ties with both political organisations.

Manuel Sarrazin, a European policy expert for the Greens, and Annette Groth, a member of Die Linke and head of a German-Greek parliamentary group, told Reuters that Berlin should repay the forced loan that Nazi Germany took from the Bank of Greece in 1942. Berlin and Athens should “jointly and amicably” take any other claims to the international court of justice, Sarrazin said.

Isychos said compensation could take the form of German companies investing in Greece and providing jobs. He cited the example of Krupps, which had been part of the industrial-military complex of Nazi Germany, continuing to provide services in Poland, another country that suffered greatly under the Third Reich.

Officials in Athens hinted that as “a last negotiating card” they could take the case to the international court of justice at The Hague. “We don’t want to impose on the Germans how they are going to work with us,” said the deputy defence minister. “But what we are saying is this is an open issue, an open wound, and we have to solve it.”