An advocacy group has offered Donald Trump sensitivity training to help him better understand people with disabilities after he appeared to mock a reporter.

Speaking at a rally in South Carolina on Tuesday, the Republican US presidential candidate mocked Serge Kovaleski, a reporter who has contested Mr Trump’s claims that thousands of people celebrated in New Jersey after the 9/11 attacks.

The US billionaire had previously cited an article written by Mr Kovaleski for the Washington Post in 2001 to back up his claims of the celebrations.

The article said that some people had been detained in New Jersey after they were “allegedly seen celebrating the attacks and holding tailgate-style parties on rooftops”.

Mr Kovaleski - who has arthrogryposis, a condition that affects joint movement and weakens the muscles around them - told CNN this week that he had no recollection of seeing “thousands” of people celebrating.

He said: “We did a lot of shoe leather reporting in and around Jersey City and talked to a lot of residents and officials for the broader story. Much of that has, indeed, faded from memory.

“But I do not recall anyone saying there were thousands, or even hundreds, of people celebrating. That was not the case, as best as I can remember.”

Following Mr Kovaleski’s comments, Mr Trump mocked the reporter on Tuesday, flailing his arms and hands as he spoke: “Now the poor guy. You ought to see this guy: ‘Uhh, I don’t know what I said. Uhh, I don’t remember.’ He’s going: ‘I don’t remember. Maybe that’s what I said.’”

Speaking with politico.com, a spokeswoman for the New York Times said: “We think it’s outrageous that he would ridicule the appearance of one of our reporters.”

A Thanksgiving Day statement from the Ruderman Family Foundation, which works to promote more inclusive policies for people with disabilities, condemned Trump.

“It is unacceptable for a child to mock another child’s disability on the playground, never mind a presidential candidate mocking someone’s disability as part of a national political discourse,” said Jay Ruderman, the foundation president. ”Our presidential candidates should be moral examples for all Americans and not disparage people with disabilities, who make up 20 percent of the American population.”

Ruderman said Trump would benefit from a “series of sensitivity training sessions” and offered to provide them.

Trump has not publicly acknowledged the incident yet but has attacked the New York Times on Twitter, calling the organisation “dopes” and criticising various business moves.

One tweet reads: “So, since the people at the @nytimes have made all bad decisions over the last decade, why do people care what they write. Incompetent!”

He also tweeted that the paper is “poorly managed” and that the paper should be “focused on good reporting and the papers [SIC]financial survival and not with constant hits on Donald Trump!”