US Senator John McCain returns to Washington on Tuesday for the first time since being diagnosed with brain cancer, his office announced, in order to take key votes on repealing Obamacare and passing sanctions on Russia.

"Senator McCain looks forward to returning to the United States Senate tomorrow to continue working on important legislation, including health care reform, the National Defense Authorization Act and new sanctions on Russia, Iran and North Korea," a statement read late Monday.

The 80-year-old McCain, recuperating after surgery in his home state of Arizona, tweeted a similar message, announcing he would be back in action in the Senate on Tuesday.

His aides announced last week that he underwent surgery to remove a blood clot from above his left eye, and tests confirmed discovery of a brain tumor known as a glioblastoma, a particularly aggressive cancer.

His vote on health care is seen to be crucial. 

Republicans hold 52 of the Senate's 100 seats. With all Democrats opposed, President Donald Trump can afford just two defectors on the health care reform vote.

Republicans are threading the needle on the bill, as three of the party's senators said last week they would vote no on the motion to proceed.

McCain has told reporters in the past that he would likely vote yes on the so-called motion to proceed, the step that allows lawmakers to begin debate on legislation.

Republican leadership and Trump himself are leaning heavily on lawmakers to agree to vote on the procedural step that allows the Senate to begin debating -- and amending -- the bill.

A vote is also expected Tuesday in the House of Representatives on a sanctions bill that will likely quickly come to the Senate.

The bill would impose tough new sanctions on Russia for its alleged meddling in the US presidential election last year and Moscow's annexation of Crimea. It would also punish Iran and North Korea for recent ballistic missile tests.

Since the diagnosis, McCain has been showered with tributes from all sides as an American original whose lifetime of public service included years as a prisoner of war in Vietnam and the 2008 Republican presidential nominee.