Republican presidential candidate Donald Trump is under fire after he appeared to ridicule a reporter with a congenital joint condition that limits movement in his arms.

At a rally in South Carolina on Tuesday, Trump defended his widely discredited claim that thousands of Muslims in New Jersey cheered as the World Trade Center collapsed on Sept. 11, 2001. He then appeared to impersonate reporter Serge Kovaleski, one of the authors of a 2001 article in The Washington Post that referred to "a number of people who were allegedly seen celebrating the attacks."

"Now, the poor guy — you ought to see the guy: 'Uh, I don't know what I said. I don't remember,'" Trump said, as he contorted his arms in an apparent imitation of Kovaleski, who suffers from arthrogryposis.

Kovaleski just a day earlier had told MSNBC's Steve Kornacki that he did "not recall anyone saying there were thousands, or even hundreds, of people celebrating."

Trump on Thursday defended his controversial behavior, claiming he was not familiar with Kovaleski’s appearance and not aware of his condition.

“I have no idea who this reporter, Serge Kovalski (sic) is, what he looks like or his level of intelligence. I don’t know if he is J.J. Watt or Muhammad Ali in his prime — or somebody of less athletic or physical ability,” Trump said in a statement released to NBC News. “If Mr. Kovaleski is handicapped, I would not know because I do not know what he looks like.”

I do not know the reporter for the @nytimes, or what he looks like. I was showing a person groveling to take back a statement made long ago! — Donald J. Trump (@realDonaldTrump) November 26, 2015

Trump claimed that he had “merely mimicked what I thought would be a flustered reporter trying to get out of a statement he made long ago.” He added he has “tremendous respect for people who are physically challenged.”

The Washington Post article cited by Trump, dated Sept. 18, 2001, included the following paragraph:

In Jersey City, within hours of two jetliners' plowing into the World Trade Center, law enforcement authorities detained and questioned a number of people who were allegedly seen celebrating the attacks and holding tailgate-style parties on rooftops while they watched the devastation on the other side of the river.

Trump has said the article is "evidence" that "thousands and thousands" of Muslims cheered the fall of the Twin Towers.

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Officials have said that did not happen, and the original story did not suggest "thousands" were celebrating, as Trump has claimed. The Washington Post said "an extensive examination of news clips from that period turns up nothing."

The New York Times, where Kovaleski is currently an investigative reporter, blasted Trump.

"We think it's outrageous that he would ridicule the appearance of one of our reporters," a spokesman for The Times told NBC News.