The United States Supreme Court has ruled that lethal injection is constitutional, in a landmark ruling set to pave the way for executions to resume after a lull of more than six months.

The judges ruled by seven to two that the risk of suffering to those executed by lethal injection did not constitute cruel and unusual punishment, which is barred under the US Constitution.

Lethal injection is the most commonly used form of execution across the US.

Some 53 people were executed in the US in 2006, after the numbers reached a peak of 98 in 1999.

Several death row inmates, led by a pair from Kentucky, had urged the Supreme Court to rule on whether lethal injection violated the constitution.

They had argued that the three-part injection method in which the first part sedates the inmate, the second paralyses the muscles and the third stops the heart, caused needless suffering in some cases.

If the execution goes according to plan, the inmate quickly loses consciousness and dies in a few minutes.

But if the anaesthesia is not properly administered, the inmate can suffer immensely.

After the Supreme Court agreed in September 2007 to hear the case, 48-year-old Michael Richard, became the last person to be executed in the US just hours later as states voluntarily placed a moratorium on all executions.

He had been convicted for the rape and murder of a woman 20 years earlier.

In her dissenting opinion, Justice Ruth Bader Ginsberg, one of just two justices to part ways with the majority, said she was not convinced that the death penalty method avoided needless suffering.

The executive director for Amnesty International USA said the high court ruling failed to resolve the broader question of whether capital punishment should be outlawed for reasons of social justice and equity.

Around two-thirds of US citizens favour the death penalty, according to the 'Death Penalty Information Centre', in a country where 3,260 detainees are presently on death row.

In December, New Jersey became the first state in 40 years to abolish the death penalty.