Image copyright Getty Images Image caption The condemned, left to right, top: Don Davis, Stacey Johnson, Jack Jones and Ledelle Lee; left to right, bottom: Jason McGehee, Bruce Ward, Kenneth Williams and Marcel Williams

Eight inmates due to be put to death over 10 days next month in Arkansas are making last-ditch bids to halt the unprecedented flurry of executions.

Lawyers for the prisoners say the "assembly-line" of four double lethal injections is unconstitutional.

Arkansas Governor Asa Hutchinson last month ordered the death row inmates to be killed before the state's execution drugs expire.

Arkansas has not executed an inmate since 2005.

No US state has put eight inmates to death in such an accelerated schedule since the US Supreme Court reinstated the death penalty in 1976.

Image copyright AP

What did the eight men do?

Bruce Ward - Convicted of 1989 strangling murder of teenage shop clerk Rebecca Doss

Don Davis - Condemned for 1990 execution-style killing of Jane Daniel as he burgled her home

Stacey Johnson - Found guilty of 1993 murder of Carol Heath, who was beaten, strangled and had her throat slit

Ledell Lee - Sentenced to death for 1993 killing of Debra Reese, who was bludgeoned with a tyre iron that her husband had given her for protection

Jack Jones - Condemned for 1995 rape and murder of accounts clerk Mary Phillips, and the nearly fatal beating of her 11-year-old daughter

Marcel Williams - Sentenced to death for 1994 rape and murder of Stacey Erickson, after kidnapping her from a convenience store

Kenneth Williams - Convicted of 1999 murder of farmer Cecil Boren during an escape from prison where Williams had been incarcerated for murdering cheerleader Dominique Hurd

Jason McGehee - Jailed for the death of 15-year-old John Melbourne, who had been his friend

In the latest legal action in the case, one of men, Stacey Eugene Johnson, asked the state's highest court on Wednesday to block his execution so evidence from his murder trial can be retested.

Attorneys for another of the convicted murderers, Bruce Ward, asked a state judge to block his execution, saying the prisoner is not mentally competent.

A group of former corrections officers wrote to the governor on Tuesday to say the pace of the executions threatens prison staff's mental health.

Two other lawsuits filed this week seek preliminary injunctions to halt the executions, arguing the inmates need time to appeal against their convictions.

Lawyers also argue that state authorities are unconstitutionally hurrying the clemency process.

"The state can show no valid reason it cannot schedule executions at a pace that would allow for meaningful review," said one of the lawsuits.

Image copyright Getty Images Image caption Arkansas Governor Asa Hutchinson last month ordered the inmates to be put to death

The convicted killers are also asking the US Supreme Court to review a ruling allowing the state to keep its source for the execution drugs a secret.

Arkansas has scheduled the executions for 17, 20, 24 and 27 April, citing a shortage of a drug used in lethal injections.

The state's supply of midazolam, a sedative used in its three-drug lethal injection cocktail, is nearing its expiration date.

The sedative has become increasingly scarce as anti-capital punishment drug-makers refuse to supply it to corrections officials.

Midazolam has been blamed for botched executions in Oklahoma, Ohio and Arizona.

Lawyers have argued that use of the sedative amounts to cruel and unusual punishment, which is banned under the US constitution.

The condemned prisoners are Bruce Ward, Don Davis, Stacey Johnson, Ledell Lee, Jack Jones, Marcel Williams, Kenneth Williams and Jason McGehee.

The state parole board has recommended clemency requests by Johnson and Lee be rejected. Further parole hearings are due this week.