Le Pen has never regretted the statement, which he first made in September 1987, he told the BFMTV television channel and RMC radio on Thursday.
"What I said was what I think, that the gas chambers were a detail of the war, unless you accept that the war was a detail of the gas chambers," he said.
The question had been "manipulated" to discredit him, he went on.
"All that is horrible, war is horrible, a mortar shell that rips open your stomach, a bomb that takes your head off, a chamber that suffocates you, it's quite dreadful."
His daughter Marine, who has been trying to clean up the party's image, was quick to dissociate herself from her father.
"Nothing new, I deeply disagree with Jean-Marie Le Pen both in form and in substance," she told the AFP news agency.
Le Pen senior has been found guilty of condoning crimes against humanity several times, having repeated the remark in 1997 on a visit to Germany, in 2008 in an interview with a magazine and in 2009 in the European parliament, where he sits as an MEP.