US President Joe Biden feels "concerned" about King Charles' cancer diagnosis.
The 81-year-old politician has revealed that he's hopeful of speaking to Charles, 75, after he was diagnosed with "a form of cancer" following a "corrective procedure" for an enlarged prostate.
Asked by reporters if he has a message for the monarch, Biden replied: "Yes, I’m concerned about him.
"I just heard his diagnosis. Hopefully I'll be talking to him soon."
The US State Department has also described the news as "incredibly sad".
Spokesperson Vedant Patel said: "Our thoughts are with the King ... and his family."
Elsewhere, Canadian Prime Minister Justin Trudeau has taken to social media to express his support for the King.
Trudeau, 52, wrote on X: "I, like Canadians across the country and people around the world, am thinking of His Majesty King Charles III as he undergoes treatment for cancer. We’re sending him our very best wishes - and hoping for a fast and full recovery."
Specific details of Charles' diagnosis haven't been revealed. However, the monarch decided to share the news "to prevent speculation and in the hope it may assist public understanding for all those around the world who are affected by cancer".
A statement from Buckingham Palace explained: "During The King’s recent hospital procedure for benign prostate enlargement, a separate issue of concern was noted. Subsequent diagnostic tests have identified a form of cancer.
"His Majesty has today commenced a schedule of regular treatments, during which time he has been advised by doctors to postpone public-facing duties. Throughout this period, His Majesty will continue to undertake State business and official paperwork as usual.
"The King is grateful to his medical team for their swift intervention, which was made possible thanks to his recent hospital procedure. He remains wholly positive about his treatment and looks forward to returning to full public duty as soon as possible."