US President Joe Biden told British Prime Minister Liz Truss on Thursday that he and his wife Jill are thinking of Queen Elizabeth amid deep concerns over her health.

"His and the first lady's thoughts are solidly and squarely with the queen today, and her family," White House national security spokesman John Kirby said.

"The president has been briefed this morning and will be updated throughout the day concerning news out of the United Kingdom."

Earlier, the British palace revealed that doctors had placed the 96-year-old monarch under medical supervision.