US Senator John McCain, a former prisoner of war and the maverick 2008 Republican presidential nominee, has been diagnosed with brain cancer, his office announced Wednesday.

The 80-year-old lawmaker from Arizona underwent surgery to remove a blood clot from above his left eye last week and tests “revealed that a primary brain tumour known as a glioblastoma was associated with the blood clot”, the Mayo Clinic, whose doctors performed the surgery in Phoenix, said in a statement released by McCain's office.

“The senator and his family are reviewing further treatment options,” the statement added, noting they may include a combination of chemotherapy and radiation.

Glioblastoma is a particularly aggressive brain tumour affecting adults. It is of the same category of cancer that led to the death of another icon of the US Senate, Ted Kennedy, who died in 2009 at age 77.

This is not McCain's first bout with cancer. Doctors removed several malignant melanomas on McCain's skin in the 1990s and 2000s, including an invasive melanoma in 2000.

Experts had said this week that McCain's latest operation suggested the possibility of a return of cancer. McCain's office said the senator “is in good spirits as he continues to recover at home with his family in Arizona”, and that he will consult with his care team about when he might be able to return to work in Washington.

The news triggered a flood of messages of support from across the political spectrum for McCain, who chairs the Senate Armed Services Committee and is in his sixth term in the upper chamber of Congress.

“Senator John McCain has always been a fighter. Melania and I send our thoughts and prayers to Senator McCain, Cindy, and their entire family,” President Donald Trump said in a statement. “Get well soon.”

'American hero'

Trump was accused of crossing the line with his criticism of McCain shortly after the billionaire businessman announced his presidential bid in 2015, disparaging the senator as “not a war hero”.

“I like people who weren't captured,” Trump said, in remarks that foretold the nastiness of the 2016 race.

But Trump is well aware of the importance of McCain's vote on health care, with the latest effort to repeal and replace the Affordable Care Act hanging by a thread.

“Plus, we need his vote,” Trump said Monday after wishing McCain well after his blood clot surgery.

“John McCain is an American hero & one of the bravest fighters I've ever known,” former president Barack Obama, who defeated McCain after a hard-fought 2008 race, wrote on Twitter. “Cancer doesn't know what it's up against. Give it hell, John.”

Senate Majority Leader Mitch McConnell quickly issued a statement saying the entire chamber was praying for their colleague and hoped to see him back soon.

“He has never shied from a fight and I know that he will face this challenge with the same extraordinary courage that has characterised his life.”

McCain, a former US Navy aviator, was shot down over Hanoi in 1967 as he conducted a bombing mission during the Vietnam War.

He was badly injured when he parachuted out, and was tortured during his five and a half years as a POW. Released in 1973, McCain won a US House seat in 1982 before winning election to the Senate four years later.

He emerged as a national security hawk, traveling the world on congressional missions to the Middle East, Asia and Europe where he reassured American allies and pricked its adversaries.

He also stood accused of heedlessly backing wars or military interventions, and once suggested US forces should remain in Iraq for 100 years.

Early this year, the octogenarian who has advocated for more aggressive US military involvement in Syria made a secret trip to the war-ravaged nation to meet with US forces and discuss the campaign against the militant Islamic State group.

Despite his hawkish positions, McCain has served the peacemaker role, too.

His scars from the Vietnam torture are visible, and yet he is the lawmaker who perhaps more than any other has guided the rapprochement between Washington and its former foe in Asia.