The White House said a botched execution in Oklahoma, which has sparked allegations the inmate underwent torture before dying, fell short of humane standards.

"We have a fundamental standard" in carrying out executions, that they need to be "carried out humanely," White House spokesman Jay Carney said.

"This case fell short of that standard."

Convicted murderer and rapist Clayton Lockett died of a heart attack more than 40 minutes after his execution was halted due to a botched lethal injection.

The situation led authorities in Oklahoma to postpone the execution of a second death row inmate.

Lockett was administered a new, untested three-drug protocol that included a sedative, an anaesthetic and a lethal dose of potassium chloride.

It would have been the state's first double execution in 80 years.

But Oklahoma Department of Corrections Director Robert Patton ordered the execution of Lockett stopped about three or four minutes after the start of the injection at 6.23pm (12.23am Irish time), citing a "vein failure".

Lockett died of a "massive heart attack" at 7.06pm (1.06am Irish time) after receiving all three drugs, spokesman Jerry Massie said.

Even though he was administered the injection, "the drugs didn't go into the system", the spokesman said.

Mr Patton then ordered a 14-day delay to the execution of Charles Warner, who was due to be executed two hours later.

Oklahoma had previously postponed the two executions in March because of a shortage of lethal injection drugs. 

But the state managed to get supplies, while changing the execution protocol, and the two inmates exhausted their appeals.

Lockett was convicted in 2000 for the rape and murder of a young woman he kidnapped, beat and buried alive.

Warner was convicted for the 1997 rape and murder of an 11-month-old girl.

Warner's lawyer Madeline Cohen had argued against the new injection combination.

She said the "experimental new drug protocol, including a paralytic" would make "it impossible to know whether the executions will comport with the Eighth Amendment's ban on cruel and unusual suffering".

"Despite repeated requests by counsel, the state has refused, again and again, to provide information about the source, purity, testing and efficacy of the drugs to be used.

"It's not even known whether the drugs were purchased legally."

Both Lockett and Warner had argued they had the constitutional right to know the composition and origin of any drugs used in the lethal injection.

In a judicial twist, Oklahoma's Supreme Court had first suspended the executions in order to resolve the controversy, but then two days later reversed itself, saying the men had no more right to information on drugs than they would for the electric chair.

Since European manufacturers began refusing to sell the most commonly used anaesthetic - pentobarbital - for human executions, several US states have found themselves facing shortages.

They are now seeking an alternative, which has led to an increase in court cases over the issue.