The White House said an investigation had been launched into whether the moving stairway was stopped on purpose to humiliate the US president, who later bashed the global body in his speech to the UN General Assembly in New York.

"If we find that these were UN and staffers who were purposefully trying to trip up, literally, trip up the President and the First Lady of the United States, well, there better be accountability for those people, and I will personally see to it," Press Secretary Karoline Leavitt said on Fox News Tuesday evening.

A UN spokesman said the mishap happened because someone in front of Trump accidentally set off a safety mechanism on the escalator, causing it to shut down.

In a statement earlier in the day, Leavitt pointed to a report in the Times of London newspaper on Sunday saying that UN staff members had joked that they would turn off the escalators and "tell him they ran out of money" amid sweeping US funding cuts.

Several other White House figures and conservative commentators also speculated about a plot against Trump.

Footage showed the 79-year-old president and First Lady Melania Trump getting on the escalator at UN headquarters, before it stopped with a lurch after a short distance.

Trump's bad luck continued when his teleprompter was not working for the start of the speech.

"Whoever is operating this teleprompter is in big trouble," said Trump.

He then went on to -- apparently jokingly -- link the two incidents to what he said were the UN's multiple failings, including a lack of support for his peace efforts in a series of conflicts.

"I ended seven wars, dealt with the leaders of each and every one of these countries, and never even received a phone call from the United Nations," Trump said.

"All I got from the United Nations was an escalator that, on the way up, stopped right in the middle. If the first lady wasn't in great shape, she would have fallen, but she's in great shape. We're both in good shape."

He added: "These are the two things I got from the United Nations, a bad escalator and a bad teleprompter. Thank you very much."

But the United Nations insisted there were simple explanations for it all.

"A subsequent investigation, including a readout of the machine's central processing unit, indicated that the escalator had stopped after a built-in safety mechanism on the comb step was triggered at the top of the escalator," Stephane Dujarric, a spokesman for UN Secretary-General Antonio Guterres, said in a statement.

A videographer filming the US delegation's arrival "may have inadvertently triggered the safety function described above," the statement continued.

The escalator was "reset" and was soon back in operation, Dujarric earlier told AFP.

"Regarding the teleprompter, we have no comment since the teleprompter for the US president is operated by the White House," Dujarric said.

AFP reporters said escalators at UN headquarters in New York were frequently out of order.