WASHINGTON - The U.S. Supreme Court on Thursday declined to place new limits on sentences of life in prison without parole for juvenile offenders, ruling against a Mississippi man convicted of killing his grandfather at age 15 in a case testing the Constitution's Eighth Amendment ban on cruel and unusual punishment.

The justices in a 6-3 ruling rejected arguments by the inmate, Brett Jones, that his sentence of life in prison with no chance of parole violated the Eighth Amendment because the judge in his trial had not made a separate finding that he was permanently incorrigible. The court's six conservatives were in the majority, with the three liberal members dissenting.

Jones, now 31, was convicted of fatally stabbing his grandfather in 2004 in a dispute involving the boy's girlfriend.

The ruling, authored by Justice Brett Kavanaugh, marked the end of the court's recent run of decisions that put limits on life sentences without parole for juvenile offenders. The court has moved rightward with a 6-3 conservative majority after the addition since 2017 of three justices appointed by former President Donald Trump.

Kavanaugh said it was the responsibility of states - not courts - to "make those broad moral and policy judgments" about juvenile sentencing reform.

In a scathing dissenting opinion, liberal Justice Sonia Sotomayor said the court effectively gutted previous rulings that imposed new restrictions on juvenile sentencing.

"The court is fooling no one," Sotomayor wrote, saying the court was downplaying the impact of the ruling.

In a 2012 ruling, the Supreme Court had decided that mandatory life sentences without parole in homicide cases involving juvenile offenders represented cruel and unusual punishment. The court had previously ruled that juveniles could not be executed and only juveniles accused of murder could be subjected to life sentences without the possibility of parole.

In 2016, the justices decided that the 2012 ruling applied retroactively, meaning that convicted criminals imprisoned years earlier could then argue for their release.

Of the 50 states, 29 allow life sentences without parole for juveniles while 20 states have no prisoners serving such sentences, according to the Sentencing Project, a nonprofit group that supports sentencing reform.

(Reporting by Lawrence Hurley; Editing by Will Dunham) ((lawrence.hurley@thomsonreuters.com; Twitter: @lawrencehurley; +1 202-809-3080))