The Anti-Defamation League on Thursday called for Donald Trump's oldest son to apologize for making what appeared to be a Holocaust-themed joke.

In an interview with a Philadelphia radio station on Wednesday, Donald Trump Jr. accused the Democrats of rigging their primary system.

"The media has been her No. 1 surrogate in this," he said in an interview on 1210 WPHT radio with host Chris Stigall.

"Without the media, this wouldn't even be a contest, but the media has built her up. They've let her slide on every indiscrepancy, on every lie, on every DNC game trying to get Bernie Sanders out of this thing. If Republicans were doing that, they'd be warming up the gas chamber right now."

Evan McMullin tweet

(Twitter)

The ADL, an international Jewish advocacy organization, tweeted Thursday that "trivialization of the Holocaust and gas chamber is NEVER okay."

The group then tweeted that its members hoped Trump Jr. understood the "insensitivity and hurt" caused by his joke and asked for an apology.

The Trump campaign did not immediately respond to a request for comment.

With files from CBC News