Former US president Barack Obama took on a new high-profile role as Father Christmas on Wednesday for a surprise visit to sick children in Washington.

Sporting a festive Santa hat and armed with a sack of gifts, he delighted young patients at Children's National Hospital with gifts and hugs.

"I just want to say thank you to all of you guys," Obama told staff, who greeted him with rapturous cheers, in a video he shared on his Twitter account.

"We've had the chance to talk to some of the wonderful kids and their families," he added.

"As the dad of two girls, I can only imagine in that situation to have nurses and staff and doctors and people who are caring for them, and looking after them... that's the most important thing there is."

The 44th president of the United States still lives in Washington, where he last year dressed up as Santa to visit middle school students at a Boys & Girls Club in the capital.

US First Lady Melania Trump also made a trip to the hospital last week, reading patients a Christmas story during her visit.

She then gave each child a copy of the book, which was about a bullied Christmas ornament.

Melania did not wear a Santa hat for the visit, instead opting for a red-and-white coat and candy-cane striped stilettos.