Ruth Bader Ginsburg, the 87-year-old anchor of the US Supreme Court's liberal faction, has been hospitalised for a suspected infection, the court said Tuesday.
Ginsburg was admitted to Johns Hopkins Hospital in Baltimore on Tuesday after first going to her regular Washington hospital with fever and chills.
She underwent an endoscopic procedure "to clean out a bile duct stent that was placed last August," the court said in a statement.
Ginsburg "is resting comfortably and will stay in the hospital for a few days to receive intravenous antibiotic treatment," it added.
One of four liberals among the court's nine justices, Ginsburg's health is closely watched as Donald Trump's administration seeks an opportunity to appoint a new justice who would tilt the court solidly in a conservative direction, potentially shifting US law and social policy for decades.
Supreme Court justices serve until they die or voluntarily retire, and Ginsburg has clung to her position despite her age, aware that if she leaves it could change the landscape of US justice.
Together with the centrist Chief Justice John Roberts acting as a swing voter, the four liberals have prevented a reversal of longstanding abortion rights, stronger executive powers for the president and greater involvement of religion in public life.
Asked in a White House press conference about Ginsburg, Trump indicated he had not hear the news.
"I wish her the best," he said. "She's actually given me some good rulings."
Trump has welcomed some Supreme Court rulings, but more often over the past three years it has dealt his administration a number of setbacks, on his immigration policies and, recently, his effort to prevent his finances from being made public.
Both Trump's Republicans and Democrats have made the balance of the court a key election issue.
Trump, who has already appointed two justices to the court, said in June that he was planning to release a list of potential candidates for the next vacant seat if he is reelected.
"If given the opportunity, I will only choose from this list, as in the past, a Conservative Supreme Court Justice," he tweeted.
Democrats for their part have sought to mobilise voters, warning that Trump should not be allowed to name another justice.
Hugely popular with Democrats, Ginsburg has become a feminist hero and an unintentional social media icon fondly known as "The Notorious RBG," a riff on rapper The Notorious B.I.G.
She has been hospitalised several times in recent years, including for two days in May to be treated for a benign gallbladder infection.
But her hospital stays have always seen her actively participating in court activities by teleconference.
It was not clear if she took part in court discussions overnight Monday on emergency petitions over the resumption of federal executions as a convicted murdered awaited on death row in an Indiana prison.
But she joint dissents by the opposing minority when the court ruled five to four in favour of going ahead.
Hours later the murderer, Daniel Lee, was put to death, in the first federal execution in 17 years.