US President Joe Biden told Liz Truss he was “keeping the Queen and her family in his thoughts”, the White House said.
The president and the Prime Minister were taking part in a video conference about the war in Ukraine.
“President Biden also conveyed to Prime Minister Truss that he was following reports about Queen Elizabeth’s health and that he and the First Lady are keeping the Queen and her family in their thoughts,” a White House statement said.
Joe Biden and Liz Truss
PA
The Queen
Kirsty O'Connor
Mr Biden's comments come hours after a Buckingham Palace statement said doctors were "concerned" over Her Majesty's health.
The statement read: “Following further evaluation this morning, The Queen’s doctors are concerned for Her Majesty’s health and have recommended she remain under medical supervision.
"The Queen remains comfortable and at Balmoral.”
Several members of the Royal Family have travelled to Balmoral to visit the Queen.
Among those to have made the trip include Prince Charles, Camilla, Duchess of Cornwall, Prince William, Prince Andrew, Prince Harry and the Earl and Countess of Wessex.
Speaking earlier on Thursday, Ms Truss said: “The whole country will be deeply concerned by the news from Buckingham Palace this lunchtime.
“My thoughts – and the thoughts of people across our United Kingdom – are with Her Majesty The Queen and her family at this time.”
While Labour leader Sir Keir Starmer said he is “deeply worried” by the news from Buckingham Palace and he is hoping for the Queen’s recovery.
He tweeted: “Along with the rest of the country, I am deeply worried by the news from Buckingham Palace this afternoon.
“My thoughts are with Her Majesty the Queen and her family at this time, and I join everyone across the United Kingdom in hoping for her recovery.”