An inmate put to death in the US state of Arizona took nearly two hours to die, prison officials said, in the latest controversy to hit America's lethal injection execution regime.

The attorney representing Joseph Wood, convicted in 1989 of the murders of his girlfriend and her father, said his client died an agonising death after being injected with a cocktail of medications that were supposed to quietly end his life.

"It took Joseph Wood two hours to die, and he gasped and struggled to breathe for about an hour and 40 minutes," attorney Dale Baich said in a statement released after the execution.

During that time, his lawyers filed an unsuccessful emergency appeal to multiple federal courts that sought to have the execution halted and their client given life-saving medical treatment.

The appeal, which said the procedure violated Wood's constitutional right to be executed without suffering cruel and unusual punishment, was denied by Justice Anthony Kennedy of the US Supreme Court.

Wood died at 3:49pm local time - almost two hours after officials began injecting him with drugs that should have taken his life in a matter of minutes, prison officials said.

His attorney said he had been injected with a mixture of two drugs - midazolam combined with hydromorphone - an experimental cocktail that "failed".

"He gasped and struggled to breathe for about an hour and 40 minutes," Mr Baich said.

"Arizona appears to have joined several other states who have been responsible for an entirely preventable horror: a bungled execution. The public should hold its officials responsible."

Wood, 55, was sentenced to die for the 1989 shooting deaths of his 29-year-old former girlfriend Debbie Dietz and her father Gene, 55.

Governor says justice was done

Arizona governor Jan Brewer expressed concern over how long the execution took and ordered the state's Department of Corrections to conduct a full review, but said justice had been done and that the execution was lawful.

"One thing is certain, however, inmate Wood died in a lawful manner and by eyewitness and medical accounts he did not suffer," the Republican governor said in a statement.

"This is in stark comparison to the gruesome, vicious suffering that he inflicted on his two victims, and the lifetime of suffering he has caused their family."

An Arizona Republic journalist who witnessed the execution said he counted the inmate gasping for breath about 660 times.

"I just know it was not efficient," said the reporter, Michael Kiefer. "It took a long time."

Charles Ryan, director of Arizona's Department of Corrections, said protocol was followed and the execution was monitored by a team of licensed medical professionals.

He said Wood was "fully and deeply sedated" five minutes after the drugs began to be administered, and the medical team reaffirmed that Wood remained deeply sedated seven more times before he was pronounced dead.

In a statement, Mr Ryan said that apart from snoring, Wood "did not grimace or make any further movement."

The Pima County Medical Examiner will conduct an independent autopsy, he said, and a toxicology study was also requested.

Anti-death penalty campaigners expressed horror over the drawn-out death.

Cassandra Stubbs, director of the American Civil Liberties Union's Capital Punishment Project, said Arizona had broken constitutional rights and the bounds of basic decency.

"It's time for Arizona and the other states still using lethal injection to admit that this experiment with unreliable drugs is a failure," she said in a statement.

Diann Rust-Tierney, executive director of the National Coalition to Abolish the Death Penalty, said Wood's execution had been shocking, cruel and entirely predictable.

"Americans have had enough of the barbarism," she said.

Inmate filed court challenge to stop execution

Wood had filed a court challenge to his execution, demanding to know more about the state's lethal injection method, the executioner's qualifications and the manufacturer of the lethal drugs.

His lawyers had also wanted to know the qualifications of the medical staff conducting the execution.

His final legal recourse, the US Supreme Court, on Tuesday refused to hear his appeal, clearing the way for officials in Arizona to proceed with his execution.

Wood is one of six death row inmates to resort to the courts to seek greater transparency about the method being used to put them to death, amid concern about the efficacy of the lethal drug protocol, especially following a recent botched execution in Oklahoma which saw an inmate appear to suffer before he died.

They argued that secrecy surrounding the drugs used in other botched executions in Ohio and Oklahoma violated their rights.

The Death Penalty Information Centre charged that the Supreme Court's decision "allows drug secrecy to continue".

The Ninth US Circuit Court of Appeals had placed a hold on Wood's execution, demanding more information from Arizona.

Individual US states may choose whether they will implement the death penalty.

Those which carry out executions have relied increasingly on compounding pharmacies, which lack federal approval, since European drugmakers refused to provide products used to execute inmates.

In January, convicted rapist and murderer Dennis McGuire was put to death in Ohio using a sedative-painkiller mix of midazolam and hydromorphone, the first such combination administered for a lethal injection in the US.

The execution took about 25 minutes to complete, with McGuire reportedly convulsing and gasping for breath.

Oklahoma suspended its executions for six months after the April death of convicted killer and rapist Clayton Lockett by lethal injection - a process that took 43 minutes, far more than the expected time of about 10 minutes.

The needle administering the lethal injection became dislodged and the execution was halted, but Lockett died 30 minutes later of a heart attack.

AFP/Reuters