Republican US Senator Lindsey Graham says he has tested positive for COVID-19 despite being vaccinated, has flu-like symptoms and will be self-quarantining for 10 days.

“I am very glad I was vaccinated because without vaccination I am certain I would not feel as well as I do now. My symptoms would be far worse,” Senator Graham, 66, wrote on Twitter.

The South Carolina Republican, a leading conservative voice in the US Senate, said he began experiencing flu-like symptoms on Saturday and sought medical attention on Monday morning.

He was later informed by a congressional physician that he had tested positive.

“I feel like I have a sinus infection and at present time I have mild symptoms,” Senator Graham wrote in a pair of tweets.

“I will be quarantining for 10 days.”

Tweet from @LindseyGrahamSC

His announcement comes at a crucial time for the closely divided US Senate, where Democratic leaders hope to advance President Joe Biden’s agenda in coming weeks by pushing through a roughly $US1 trillion ($A1.4 trillion) bipartisan infrastructure bill.

Senator Graham is among a group of 17 Senate Republicans who have backed the bipartisan infrastructure legislation in procedural votes.

The measure needs Republican votes to achieve the 60-vote threshold required for most legislation in the 100-seat chamber.

The Senate is split 50-50 between Republicans and Democrats, who control the chamber only because of Vice President Kamala Harris’ tie-breaking vote.

It was not clear how much contact Senator Graham has had recently with other members of the Senate.

A spokesman for Senator Joe Manchin, a leading Democrat on infrastructure, could not confirm whether Senator Graham attended a recent private gathering hosted by Manchin but added: “Senator Manchin is fully vaccinated and following the CDC guidelines for those exposed to a COVID positive individual.”

-AAP