Inmates armed with homemade knives have fought each other in a seven-hour riot over territory and money, leaving seven of them dead in the worst US prison riot in a quarter-century, officials say.

An inmate who witnessed the violence said that bodies were “literally stacked on top of each other” at the Lee Correctional Institution in South Carolina.

At least 17 prisoners were seriously injured, the state’s prisons chief Bryan Stirling said.

The first fight started in a dorm about 7:15pm on Sunday (US time) and appeared to be contained before suddenly starting in two other dorms.

Mobile phones helped stir up the trouble, and state officials are now urging the federal government to change a law and allow them to block the signals from prisoners’ phones.

“These folks are fighting over real money and real territory while they’re incarcerated,” Mr Stirling said at a news conference.

No prison guards were hurt.

Mr Stirling said guards followed protocol by backing out and asking for support.

It took several hours to restore order, but once a special SWAT team entered, the inmates gave up peacefully, he said.

The prisoner who saw the riot exchanged messages with the Associated Press on the condition of anonymity because he is not allowed to have a mobile phone and feared retribution from other inmates.

He said he saw several attackers taunt a rival gang member who was badly injured.

“I just saw three dead on the sidewalk outside of my unit. One guy is still alive and breathing, but just barely.”

The riot was the latest violence in the South Carolina prisons system, where at least 13 other inmates have been killed by fellow prisoners since the start of 2017.

It was the most inmates slain in a single riot in the US since nine prisoners and a guard died in 1993 at the Southern Ohio Correctional Facility, said Steve Martin, a consultant who helps the federal government monitor prison systems.

The inmate who spoke to media said many cell-door locks were already broken before the riot and that he and other prisoners roamed around freely at the prison in Bishopville, located 64 kilometres east of Columbia.

Hours after the violence started, no correctional officers or medical personnel attended to the dead or dying, he said.

“The COs [corrections officers] never even attempted to render aid, nor quell the disturbance,” he said. “They just sat in the control bubble, called the issue in, then sat on their collective asses.”

‘We went in as soon as we thought it was safe’

Corey Scott, Eddie Casey Gaskins, Raymond Angelo Scott, Damonte Rivera, Michael Milledge, Cornelius McClary and Joshua Jenkins were killed in the riots. Photo: AP

Mr Stirling said the response teams entered as quickly as they could.

“We … went in as soon as we thought it was safe for our staff,” he said.

Most of the slain inmates were stabbed with homemade knives or slashed, while the remainder appeared to have been beaten, Lee County Coroner Larry Logan said.

Mr Stirling said an investigation would determine if any other type of weapon was used.

The slain were serving anywhere from 10 years to life in prison and their crimes included murder, burglary and trafficking crack cocaine.

The youngest was 28 years old while the oldest was 44.

The injured inmates required medical attention outside the prison, which made it more difficult for authorities to restore order, Mr Stirling said.

The coroner said when he arrived it was a chaotic scene of fighting everywhere.

Mr Logan said the state-run Lee Correctional Institution, like most other South Carolina prisons, had struggled to find enough workers.

But he said he did not believe anything could have been done once things got that far out of control.

“If everybody has an uprising, you are always going to be understaffed,” he said.

The maximum-security facility in Bishopville houses about 1500 inmates and there were 44 guards there when the first fight started.

“It’s an incredibly bad day in South Carolina,” said Senator Gerald Malloy, whose district includes Lee Correctional. “We failed. That’s it.”

Two officers were stabbed at Lee Correctional in 2015. More recently, an inmate held a guard hostage for 90 minutes in March and another killed a fellow prisoner in February.

Governor Henry McMaster commended Mr Stirling’s response and said he was outraged the state was not allowed to jam mobile phone signals in prisons.

“There are prisons around the country — state prisons, federal prisons all — that would be safer with this jamming,” he said.

Mr Stirling said officials planned to meet next month with the phone industry to talk about solutions, but “until that’s done, the folks that are incarcerated are going to continue their criminal ways from behind bars”.

-ABC