The United States' top court ruled Wednesday that the lethal three-drug cocktail used in executions is legal, a ruling that will allow executions in the country to resume.

Executions in the United States were stayed in September when the Supreme Court agreed to hear the appeal from two Kentucky death row inmates.

In Wednesday's ruling, the nine-judge panel upheld the use of the injection, rejecting the appeal of Ralph Baze and Thomas Clyde Bowling — both convicted of double homicide and sentenced to death — who argued the lethal injections violated the country's constitutional ban on cruel and unusual punishment.

The two initially sued in 2004, with the argument the drug cocktail can inflict unnecessary suffering. They asked instead to be executed using a single drug administered in a large enough dose to kill.

The three-drug injection method is meant first to render a prisoner unconscious, then unable to move, then kill. States began using this method in 1978 as an alternative to electrocution, the gas chamber, hanging and shooting. However, recently, there have been botched injections in Florida and Ohio in which inmates took much longer to die.

Critics of the method say that if the first shot is ineffective, the other two injections can cause terrible pain.

A Kentucky judge first upheld the use of the injection, followed by a similar ruling by the state's Supreme Court, which led to the top court appeal.

The appeal by Baze and Bowling was overturned Wednesday by a 7-2 vote.

"We, too, agree that petitioners have not carried their burden of showing that the risk of pain from maladministration of a concededly humane lethal injection protocol, and the failure to adopt untried and untested alternatives, constitute cruel and unusual punishment," Chief Justice John Roberts wrote in the decision.

There was no immediate indication on when executions would resume, but prosecutors in several states said they would seek new execution dates if the court ruled favourably in the Kentucky case.

Forty-two people were executed last year among more than 3,300 people on death row across the country. Another roughly two dozen executions did not go forward because of the Supreme Court's review, death penalty opponents said.

With files from the Associated Press