The 80-year-old senator underwent surgery in Phoenix last Friday to remove a blood clot over his left eye, which is now known to be associated with the tumor.

"The Senator and his family are reviewing future treatment options with their Mayo Clinic team. These options may include a combination of chemo and radiation," his office said in the statement.

"The senator's doctors say he's recovering from his surgery amazingly well," the note added.

McCain's surgery forced Republicans to delay the vote on the new health bill, envisaged for this week in the upper house, because of the announced absence in Washington from the senator, although until today it was not known of the seriousness of his state.

McCain fought in the Vietnam War, where he was a prisoner of war for more than five years, since 1987 he is a senator from Arizona and in 2008 he lost the White House election to Barack Obama.

US President Donald Trump said in a statement that McCain "has always been a fighter," wished him a "speedy recovery" and sent his "prayers" and those of First Lady Melania to the Senator and his family.

His predecessor, Barack Obama, noted that McCain is an American "hero" and one of the "bravest fighters" he has ever known: "Cancer does not know what it’s up against. Give it hell, John.”

Obama's vice president Joe Biden, whose eldest son, "Beau," died in 2015 of a brain tumor, said McCain is a "strong" man who "has gone through a lot of difficulties" and expressed confidence that he will "beat this”.