WASHINGTON, United States (AFP) — US Senator John McCain, who is fighting an aggressive form of brain cancer, was in a military hospital Wednesday to treat “normal side effects” of his therapy, his office said.

The 81-year-old former prisoner of war and 2008 Republican presidential nominee was diagnosed in July with a brain tumour known as a glioblastoma, after doctors found a blood clot over his left eye during a routine checkup.

McCain underwent surgery at the time in his home state of Arizona, and has been treated with radiation and chemotherapy.

“Senator McCain is currently receiving treatment at Walter Reed Medical Centre (in Bethesda, Maryland) for normal side effects of his ongoing cancer therapy,” his office said in a statement. “As ever, he remains grateful to his physicians for their excellent care, and his friends and supporters for their encouragement and good wishes,” the statement added.

“Senator McCain looks forward to returning to work as soon as possible.”

A day earlier McCain received an MRI scan at the National Institutes of Health, also in Bethesda, his office said.

He will continue to receive targeted radiation and chemotherapy treatments at NIH “while maintaining a regular work schedule in the United States Senate,” it added.

McCain, who has been critical of President Donald Trump, proved to be a crucial vote on Trump’s effort to repeal and replace the Affordable Care Act health law.

The veteran senator, along with two other Republicans, voted against the bill, dooming the measure in a deep embarrassment to the White House.

He is again being closely watched for his vote on the current major bill before Congress, an overhaul of the tax code.

McCain voted for the measure last month, but it is expected to come before the chamber, where Republicans hold a narrow 52-48 majority, again in the coming weeks for a vote on final passage. No Democrats are currently supporting the measure.

Should McCain’s return be delayed, prospects for the tax overhaul would narrow substantially.