White House Chief of Staff John Kelly, left, reacts as he and first lady Melania Trump listen to U.S. President Donald Trump speak during the 72nd session of the United Nations General Assembly - AP

John Kelly, Donald Trump's chief-of-staff, appeared to go through an "existential crisis" on Tuesday, social media users joked, as he listened to the president's address to the UN General Assembly.

Mr Trump used his maiden address to the world's leaders to escalate his standoff with North Korea over its nuclear challenge, threatening to "totally destroy" the country of 26 million people and mocking its leader, Kim Jong-un, as a "rocket man."

A great and important day at the United Nations.Met with leaders of many nations who agree with much (or all) of what I stated in my speech! — Donald J. Trump (@realDonaldTrump) September 20, 2017

It was described by one observer as a "42-minute tweetstorm" and by another as president George W. Bush's "'axis of evil' speech on steroids."

But it was the reaction of his right hand man, Mr Kelly, that became one of the main talking points on social media.

White House Chief of Staff John Kelly, left, reacts as he and first lady Melania Trump listen to U.S. President Donald Trump speak at the United Nations General Assembly Credit: AP

Seen with his head in his hand in one photograph, and looking down at the floor in another, Twitter users guessed he wasn't enjoying what he heard.

"John Kelly apparently went through some sort of existential crisis during Trump's UN speech," Kyle Feldscher, a reporter, commented.

Welcome to our world, Kelly. https://t.co/OqLElcqHYl — Scott Weinberg (@scottEweinberg) September 19, 2017

Did Kelly not know what Trump was going to say? https://t.co/LKrq6wrLck — Otillia (@o_tilli_a) September 19, 2017

John Kelly has a terrible poker face https://t.co/JIM05ybgWF — Jessica Taylor (@JessicaTaylor) September 19, 2017

John Kelly, doing his best impression of Kif Kroker's weary sigh. pic.twitter.com/rYYPHOJiqI — MAURICE LAMARCHE (@MAURICELAMARCHE) September 19, 2017

John Kelly at UNGA is all of us. pic.twitter.com/Wza8wrzdCY — Mira Rapp-Hooper (@MiraRappHooper) September 19, 2017

Others said his facial expressions summed up what everyone was thinking.

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The comments echoed the reaction to his facial expressions during Mr Trump's controversial press conference on the violence at the Charlottesville white supremacist rally, in which the president said both sides were to blame.

White House Chief of Staff Kelly checks his watch as he listens to U.S. President Trump deliver his address to the UNCredit: Reuters

My Kelly, 67, was brought in at the start of August to bring order to the White House.

Within a month, however, there were reports of tensions, with Mr Trump allegedly lashing out at the chief of staff.

Mr Kelly told colleagues he had never been spoken to in such a way during 35 years in the military, the New York Times reported.

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