US President Donald Trump has told a 7-year-old girl that believing in Santa at that age is "marginal", as he and wife Melania took phone calls from children anxious to know the whereabouts of Father Christmas.

Key points: The NORAD Tracks Santa program has become a Christmas Eve tradition

The NORAD Tracks Santa program has become a Christmas Eve tradition Mr Trump told one caller that "at 7, it's marginal" to believe in Santa

Mr Trump told one caller that "at 7, it's marginal" to believe in Santa The Trumps later attended a service at the Washington National Cathedral

In one conversation, Mr Trump asked a girl named Collman, 7, "are you still a believer in Santa?"

He listened for a moment before adding, "because at 7, it's marginal, right?"

Mr Trump listened again and chuckled before saying, "well, you just enjoy yourself."

The First Lady later tweeted that helping children track Santa was "becoming one of my favourite traditions".

Melania and Donald Trump take calls from children on Christmas Eve. ( AP/Jacquelyn Martin )

She told one caller that Santa was in the Sahara. Several minutes later, she reported that he was far away in Morocco but would be at the caller's home on Christmas morning.

The young callers were part of the North American Aerospace Defence Command (NORAD) Tracks Santa program, a Christmas Eve tradition that started after a child mistakenly called the forerunner to NORAD in 1955 asking to speak to Santa.

The program wasn't affected by the recent government shutdown and is run by volunteers at Peterson Air Force Base in Colorado using pre-approved funding.

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The Trumps later travelled to Washington National Cathedral to attend the Solemn Holy Eucharist of Christmas Eve.

The cathedral's website said the program included readings from Holy Scripture, favourite congregational hymns and seasonal choral and instrumental music as well as Holy Communion.

Passes were required.

Mr Trump most likely would have been attending Christmas services at a church near his estate in Palm Beach, Florida.

But he scrapped plans to head to Florida for the holidays after parts of the government were forced to shut down indefinitely in a budget stalemate with Congress.