Washington, March 1 (Reuters): The US Supreme Court today abolished the death penalty for juveniles, an important victory for opponents of capital punishment in the only country that gave official sanction to such executions.

In a 5-4 ruling that cited the ?overwhelming weight of international opinion,? the high court declared unconstitutional the death penalty for those under the age of 18 when they committed their crimes.

The decision in the case of a 1993 Missouri killing could affect more than 70 death-row inmates who face execution for murders done when they were 16 or 17 years old. The total US death-row population is nearly 3,500.

The court abandoned its decision 16 years ago that the execution of juvenile offenders did not violate the constitutional ban on cruel and unusual punishment.

Tuesday?s decision was another in the court?s recent history of limiting the application of the death penalty, which it had reinstated in 1976.

The Amnesty International human rights organisation, which has campaigned for years against the execution of juvenile offenders, hailed the ruling.

?Today?s ruling we see as one of the final milestones in the road to global abolition of the death penalty for crimes committed by children,? a spokeswoman for the group said in London.

Opponents of capital punishment had argued that world opinion and now a national consensus opposed the juvenile death penalty and said it should be struck down as unconstitutional.

In the court?s majority opinion, Justice Anthony Kennedy agreed. ?Neither retribution nor deterrence provides adequate justification for imposing the death penalty on juvenile offenders,? Kennedy wrote in the 25-page opinion.

?It is proper that we acknowledge the overwhelming weight of international opinion against the juvenile death penalty, resting in large part on the understanding that the instability and emotional imbalance of young people may often be a factor in the crime,? he said.

The ruling came in a case from Missouri involving Christopher Simmons, who was 17 in 1993 when he tied up a woman and threw her from a bridge to her death by drowning.

The decision overturned his death sentence.

Simmons is now a remorseful adult who works to end youth violence, said Rita Linhardt of the Missouri Ban Youth Executions Coalition, which has pursued the case for years and called the ruling a victory.

The US was the only country in the world that still gave official sanction to the juvenile death penalty, Kennedy said in his ruling.

He noted that the UN Convention on the Rights of the Child, which prohibits the juvenile death penalty, has been ratified by every country except Somalia and the US.

Only seven countries other than the US have executed juvenile offenders since 1990, he said. They are Iran, Pakistan, Saudi Arabia, Yemen, Nigeria, the Democratic Republic of Congo and China.

Since 1990, each of the seven countries has abolished capital punishment for juveniles or made public disavowal of the practice.

Chief Justice William Rehnquist and Justices Sandra Day ?Connor, Antonin Scalia and Clarence Thomas dissented.

?I would not substitute our judgement about the moral propriety of capital punishment for 17-year-old murderers for the judgements of the nation?s legislatures,? ?Connor said.

Scalia said the court?s interpretation of the US Constitution should not be influenced by other nations.