US Senator John McCain has been diagnosed with an aggressive form of brain cancer, his office has said.
The 80-year-old is said to be reviewing treatment options that may include a combination of chemotherapy and radiation.
The six-term Republican senator and 2008 presidential nominee has been recovering at home in Arizona since undergoing surgery at a clinic in Phoenix last Friday to remove a blood clot from above his left eye.
Tissue analysis since that procedure revealed that a primary brain tumour, known as a glioblastoma, was associated with the clot, his office said.
Glioblastoma is considered a grade IV tumour, the most malignant of gliomas. It can be very aggressive and spread into other parts of the brain quickly, according to the MD Anderson Cancer Center in Texas
Senator McCain's doctors said he was recovering from his surgery "amazingly well" and that his underlying health was excellent.
President Donald Trump and his predecessor Barack Obama have both offered their support.
Melania and I send our thoughts and prayers to Senator McCain, Cindy, and their entire family. pic.twitter.com/SO4XYgnyug— President Trump (@POTUS) July 20, 2017
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
Senator McCain was one of Congress' most vocal critics of Mr Obama's foreign policy, but he has also raised questions about Mr Trump.
The senator found himself to be a brief side issue in the race for the 2016 Republican presidential nomination when he criticised then-candidate Trump, who responded by saying Mr McCainwas not a war hero because he had been captured by the Vietnamese.
Senate Majority Leader Mitch McConnell called Senator McCain, the chairman of the Armed Services Committee, and a prisoner of war during the Vietnam War, a hero and said he looked forward to having him back in Washington.
"We all look forward to seeing this American hero again soon," Mr McConnell said in a statement.
While known as a fierce advocate for strong US military action overseas, Senator McCain also has a reputation for working with Democrats on issues from clamping down on campaign finance abuses to immigration reform.
This week, he called for a bipartisan approach to overhauling the US healthcare system.
His absence from Washington has complicated efforts by Mr Trump and his fellow Republicans to repeal Mr Obama's Affordable Care Act, popularly known as Obamacare and makes it difficult for Mr McConnell to gather the 50 votes he needs in a chamber where the party holds only a 52-48-seat margin.
His absence could also complicate progress toward passing the annual National Defense Authorization Act, a $700 billion piece of legislation setting policy for the Department of Defense that must pass every year.
Senator McCain, the son and grandson of admirals, was a US Navy pilot.
His plane was shot down over Vietnam in 1967 and he spent five-and-a-half years as a prisoner of war.
When he was offered release because of his father's rank, Senator McCain refused to be freed before those who had been held captive longer. He finally returned to the United States in 1973, with other prisoners of war.
One of his proudest moments as a US senator was working to pass legislation banning torture in 2015.