US President Joe Biden says he is concerned about King Charles's cancer diagnosis and hopes to speak to him soon.
He told reporters in Las Vegas on Monday that he had "just heard his diagnosis".
"I'm concerned about him," he said at a union event. "But I'll be talking to him God willing."
Buckingham Palace earlier announced that the King was being treated for cancer.
The type of cancer has not been revealed, but according to a Palace statement the King began "regular treatments" on Monday.
He "remains wholly positive about his treatment and looks forward to returning to full public duty as soon as possible", the statement added.
In a post on X later on Monday, Mr Biden said: "Navigating a cancer diagnosis, treatment, and survivorship takes hope and absolute courage. Jill and I join the people of the United Kingdom in praying that His Majesty experiences a swift and full recovery."
Mr Biden's son, Beau, died of brain cancer aged 46, and his long-time friend, Republican Senator John McCain, also died of cancer in 2018.
The King, 75, is beginning treatment as an outpatient and will pause his public events, but will continue with his constitutional role as head of state, including paperwork and private meetings.
King Charles's cancer diagnosis
The US State Department said his diagnosis was "incredibly sad news".
"Our thoughts are with the King... and his family," spokesman Vedant Patel told reporters at a regular news briefing.
Canadian Prime Minister Justin Trudeau also sent well wishes to the King, saying Canadians across the country were thinking of him. "We're sending him our very best wishes - and hoping for a fast and full recovery," he said.
Former President Donald Trump said the King was a "wonderful man who I got to know well during my presidency".
"We all pray that he has a fast and full recovery," he wrote on his social media site Truth Social.
Meanwhile, Prince Harry, who lives in California with his wife Meghan Markle, will travel to the UK to see his father, the BBC has been told.
The duke moved to the US with his wife, Meghan, Duchess of Sussex, after they stepped back as senior royals in 2020.