The incident, which quickly went viral online, took place after Biden and First Lady Jill Biden spoke by video conference with North American Aerospace Defense Command (NORAD) officials, who track Santa's travel across the globe.

After being assured of St Nick's progress, the president fielded calls from several American families.

One man identified himself as "Jared," a father of four. Biden spoke briefly with the caller's children, asking what presents they wanted for Christmas and telling them they needed to be in bed before midnight. He also remarked on how he and Jared both had sons named Hunter.

Wrapping up the call, Biden wished Jared a "wonderful Christmas." 

"I hope you guys have a wonderful Christmas as well, Merry Christmas, and let's go Brandon!" Jared replied.

"Let's go Brandon, I agree," returned Biden, without reacting to the dig. It was not immediately clear if the president had understood the reference, though Jill Biden chuckled uncomfortably and rolled her eyes.

Biden asked Jared where he was calling from, but by then the call had been disconnected. A clip of the moment spread widely on social media, with some praising the president for his calm response.

"Let's Go Brandon" is a euphemism for a vulgar insult that originated when a reporter at a NASCAR event misheard a chant of "F- Joe Biden" while interviewing driver Brandon Brown in September.

Since then it has been taken up as a political slogan by supporters of Trump, Biden's predecessor.

Trump had his own headline-grabbing Christmas Eve moment when, in 2018, he raised doubts about Santa's existence in a call with a seven-year-old boy.

"Are you still a believer in Santa Claus? 'Cuz at seven it's marginal, right?" he said.

The Santa tracker presented by NORAD dates to 1955, when a Colorado newspaper advertisement misprinted a phone number to connect children with Santa and mistakenly directed them to the military nerve center hotline.

Later on Saturday, Biden commended Americans for their strength and resilience in the face of the raging Covid-19 pandemic, urging "hope and renewal" during the holiday season.

In his first Christmas address as president, Biden praised "the enormous courage, character, resilience, and resolve in all of you who heal, comfort, teach, and protect and serve in ways big and small."

"You show there is much to gain in appreciation and gratitude for the gift of time and goodwill we share as we look out for one another," Biden said in a statement with First Lady Jill Biden. 

"Again and again, you show how our differences are precious and our similarities infinite."

Biden, a devout Catholic, went on, saying that "for the nation, we pray for the promise found in Scripture -- of finding light in the darkness, which is also perhaps the most American of things to do."

Later Saturday, the Bidens spoke to service members stationed domestically and at US bases overseas. Also present at the event was the newest addition to the first family, a German shepherd puppy named Commander.

"As your commander in chief, I wanted to take this opportunity to say thank you, thank you, thank you," Biden told soldiers based in Qatar, Bahrain, Romania and the US state of Colorado. 

"You are the solid steel spine of the nation," he said, recalling that his son Beau, who died of brain cancer in 2015, had served in Iraq.

"Jill and I know how hard it is for you" to spend a season of togetherness away from loved ones, he added.

"It's a poor substitute for what you are missing," Biden said of his presidential message.

Jill Biden, meanwhile, spoke about her father who served in the US Navy during World War II.

With Americans battered by the deadly pandemic and economic hardship exacerbated by record-high inflation, the presidential couple have focused on end-of-year traditions -- pardoning a turkey on Thanksgiving and decorating the White House for Christmas -- to send messages of hope and joy.

But with the country rocked by deep political divisions, even a Christmas Eve greeting turned sour for Biden Friday, when a man to whom he was wishing a merry Christmas during a video call from the White House, launched at him with an insult that is popular among supporters of Biden's predecessor Donald Trump.