A lawyer says he will ask the US Supreme Court to overturn the 12-year prison sentence given to an African American man in Mississippi for carrying his mobile phone into a jail cell after he was arrested on a misdemeanour charge.

The Mississippi Supreme Court said on Thursday that it will not reconsider its earlier decision to uphold the sentence of Willie Nash.

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Southern Poverty Law Center lawyer Will Bardwell told The Associated Press news agency that an appeal to the nation's high court is his next step.

Critics have slammed the sentence as an example of racial injustice. When the Mississippi Supreme Court initially upheld Nash's sentence in January, nationally syndicated Miami Herald columnist Leonard Pitts wrote that Nash was "the latest victim of a 'just us' system that promiscuously discards black life".

Pitts urged readers to call Mississippi Governor Tate Reeves and tell him to "let my people go".

Within days of the January ruling, Bardwell filed another appeal asking the Mississippi high court to reconsider.

In rejecting the request on Thursday, justices revised a portion of their analysis about previous court decisions on sentencing but did not change the outcome for Nash.

The sentence Nash received in August 2018 for possessing a cellphone in jail is longer than Mississippi courts would impose for second-degree arson or poisoning someone with the intent to kill, Bardwell wrote in his January appeal.

Bardwell also wrote that there was no proof Nash was searched for a cellphone before being booked into jail. The appeal said Mississippi is one of only three states where a 12-year sentence for having a cellphone in jail is even possible, and research found no cases of such a long sentence being given in the other two states - Arkansas and Illinois.

Nash did not seek to overturn his conviction, but argued that the sentence was so grossly disproportionate that it violates the constitutional ban on cruel and unusual punishment.

A 2012 Mississippi law sets a sentencing range of three to 15 years for inmates found with deadly weapons, cellphones or components of cellphones in state jails and prisons.

"Though harsh, Nash's sentence falls within the statutory range," Justice James Maxwell wrote in January.

The only African American justice on Mississippi's nine-member court, Leslie King, joined in the unanimous ruling in January but wrote that the prosecutor and trial judge could have avoided punishing Nash entirely.

Corrections officials have said for years that contraband cellphones are a problem. During an outbreak of violence that left five inmates dead and an undisclosed number of others injured in January, inmates shared cellphone photos and videos that showed prisoners sleeping on the floor of a crowded cell and smoke filling a corridor and cells at the Mississippi State Penitentiary at Parchman.