Republican Convention 2016 Cleveland
Donald Trump Jr raises his fist after speaking at the Republican National Convention. Jim Watson/AFP

Donald Trump's oldest son made a quip about "warming up the gas chamber" during an attack on the press for what he characterised as being too soft on Hillary Clinton.

The American Anti-Defamation League quickly issued two Tweets condemning the remark, with one admonishing that "trivialization of the Holocaust and gas chambers is NEVER okay" and later calling for a retraction.

Donald Trump Jr made his remark during a radio talk show on Philadelphia's WPHT while complaining about what he called media favouritism toward Clinton.

"They've let her slide on every in-discrepancy, on every lie. If Republicans were doing that, they'd be warming up the gas chamber right now," he said.

He later insisted to NBC that he stood by his point, but that he meant to refer to executions rather than the Holocaust. He said he normally uses the phrase "electric chair" to make the same point.

Gas chambers are used in only three states with capital punishment. The last person to be executed in a gas chamber was killed in Arizona in 1999. Clinton's campaign manager John Podesta called the media interpretation of the words a "smear." It's not the first time Donald Trump Jr has been accused of neo-Nazi tolerance.

Pepe the Frog
US white nationalist 'mascot' Pepe the Frog (in a blond wig) smiles over Donald Trump's shoulder in an Instagram photo posted by Donald Trump Jr Instagram

After Clinton called some Trump supporters a "basket of deplorables" (and later apologised), Donald Jr posted an Instagram meme showing him, his brother Eric and other supporters posing with his dad and presented as "The Deplorables" in the manner of the heroic movie "The Expendables." Except that the Instagram also featured the cartoon Pepe the Frog (in a Trump-like blond wig), who has become a mascot of the white nationalist alt-right movement in the US.

In March 2016, Donald Jr gave an interview to James Edwards, host of the white supremacist radio show Political Cesspool which has featured Holocaust deniers. Edwards has also proclaimed that "slavery was the greatest thing" to happen to African Americans.

On another issue in the president race, Trump was asked why his father hadn't yet released his tax returns. Instead of repeating his father's argument that he's under audit, his son suggested that the public would scrutinise them, which would "detract" from his father's "main message."

Republican House Speaker Paul Ryan has called on the candidate to release his returns.