US senator and former Republican presidential candidate John McCain has been diagnosed with aggressive brain cancer.

It was found during recent surgery near his eye to remove a blood clot. 

Reports suggest the 80-year-old was given the diagnoses following pathology tests after surgery.

Mr McCain's daughter, Meghan McCain, released a statement on her Twitter saying the news of her father's illness has affected everyone in the McCain family. 

"My grandmother, mother, brothers, sister and I have all endured the shock of the news and now we live with the anxiety about what comes next. 

"It is an experience familiar to us, given my father's previous battle with cancer - and it is familiar to the countless American families whose loved ones are also stricken with the tragedy of disease and the inevitability of age." 

Ms McCain, co-host on Fox News' OutNumbered, said if the family could ask for anything it would the prayers of those who understand this all too well.

"We would be so grateful for them," she wrote.

"It won't surprise you to learn that in all this, the one of us who is most confident and calm is my father. He is the toughest person I know. The cruelest enemy could not break him. The aggressions of political life could not bend him. Cancer may afflict him in many ways, but it will not make him surrender. Nothing ever has."

Ms McCain called her father "one of the greatest Americans of our age."

"But to me he is something more, he is my strength, my example, my refuge, my confidante, my teacher, my rock, my hero - my dad,"  she wrote, signing off with her initials, 'MMM'. 

Statement regarding my father @SenJohnMcCain: pic.twitter.com/SMte9Hkwkq

— Meghan McCain (@MeghanMcCain) July 20, 2017

The senator and his family were reviewing his treatment options, including a combination of chemotherapy and radiation, the Mayo Clinic of Phoenix said in a statement on Wednesday.

Former US president Barack Obama tweeted his well wishes to Mr McCain. 

"John McCain is an American hero & [sic] one of the bravest fighters I've ever known. Cancer doesn't know what it's up against. Give it hell, John."

John McCain is an American hero & one of the bravest fighters I've ever known. Cancer doesn't know what it's up against. Give it hell, John.

— Barack Obama (@BarackObama) July 20, 2017

McCain, a former Navy pilot who was shot down over Vietnam and spent five and a half years as a prisoner of war, was the GOP's presidential nominee in 2008.

The Republican lawmaker announced on Saturday that he would spend this coming week recuperating in Arizona.