The White House on Wednesday defended United States President Donald Trump’s tweet taunting North Korea leader Kim Jong-un and his country’s nuclear power.

In response to a question on whether Americans should be concerned about Trump’s mental fitness, as he appeared to take Kim’s threats of having a nuclear button on his desk lightly, Press Secretary Sarah Sanders said, “I think the president and the people of this country should be concerned about the mental fitness of the leader of North Korea.”

Sanders said Kim had repeatedly threatened the US and tested missiles for years. “And this is a president who’s not going to cower down and he’s not going to be weak, and is going to make sure that he does what he’s promised to do, and that’s stand up and protect the American people,” she said.

On Tuesday, in response to Kim’s New Year’s Day speech, in which he said “the nuclear button is always on my table”, Trump said on Twitter that his button was “much bigger and more powerful”.

“Will someone from his depleted and food starved regime please inform him that I too have a Nuclear Button, but it is a much bigger & more powerful one than his, and my Button works!” Trump had tweeted.

Trump’s tweet is the latest in a long war of words between the two leaders after North Korea conducted a string of nuclear tests in 2017.

Former Vice President Joe Biden criticised Trump for the tweet and called it “dangerously cavalier”. Biden told CNN, “I think it shows really poor judgement for the president to perform the way he does.” Biden said he was disappointed that Trump has not become more presidential. “This is not a game. This is not about, you know, can I puff my chest out.”

Senator Edward Markey said Trump’s tweet “bordered on presidential malpractice, needlessly deepening a crisis and squandering a fresh opportunity to attempt diplomacy”, Reuters reported.