Arizona Senator John McCain has been diagnosed with a brain tumour after doctors removed a blood clot above his left eye last week.

The 80-year-old Republican has glioblastoma, an aggressive cancer, doctors at the Mayo Clinic in Phoenix said.

"On Friday, July 14, Senator John McCain underwent a procedure to remove a blood clot from above his left eye at Mayo Clinic Hospital in Phoenix," his office said in a statement on Wednesday.

"Subsequent tissue pathology revealed that a primary brain tumour known as a glioblastoma was associated with the blood clot."