Former President Barack Obama delighted patients at the Children's National Medical Center of Washington as he donned a Santa hat and delivered gifts.

Obama thanked the staff for their hard work during the holidays describing it as a time that is "tough for folks".

"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."

He visited individual patients' rooms as well as one of the hospital's playrooms during his 90-minute visit. Word of Obama's arrival spread quickly throughout the hospital with staff gathering and singing an impromptu carol.