Amnesty International, in a report published on Wednesday, said that in the past decade two-thirds of known executions of under-age offenders were carried out in the US.
The organisation criticised the US Supreme Court for ruling recently that the execution of mentally disabled criminals was a "cruel and unusual punishment" while continuing to permit the execution of young offenders.
It observed that, of the United Nations' 190 member states, only the US and Somalia failed to ratify the Convention on the Rights of the Child, which bans such executions.
"By allowing the execution of child offenders the US undermines its own claims to be a progressive force for human rights," the organisation said.
"It is time for the US to come in from the cold."
International pressure
Much of the criticism levelled towards the US for its execution of younger offenders has focussed on the claim that a disproportionate number of those scheduled to die are black.
Death penalty USA
38 states have the death penalty
More then 3,700 inmates are on death row
Execution can be by hanging, electrocution, gassing, firing squad or lethal injection
Amnesty International said that all three child offenders executed this year in the US were black.
They included the high-profile case of Napoleon Beazley, who was 17 when he shot and killed a man while stealing his car.
Last May, Beazley was executed by lethal injection in Texas, despite international pressure and a last-minute appeal to the US Supreme Court.
Meanwhile the US has carried out its 800th execution since the death penalty was reinstated in 1976.
Rex Mays, convicted in 1992 for stabbing two young girls to death, was executed, also by lethal injection in Texas, the leading state for executions.
Growing debate
Amnesty International's report comes at a time of great debate in the US regarding the death penalty, increased by several prominent legal rulings that have questioned key aspects of the system.
Execution factfile
2001 - 66 executions
1951 - 105 executions
101 people released from death row since 1973
66% of Americans support the death penalty
81% take place in southern states
On Tuesday, a federal judge in Vermont declared the federal death penalty unconstitutional, the second such ruling in less than three months.
US District Judge William Sessions said that the Federal Death Penalty Act of 1994 did not protect a defendant's constitutional rights, including the right to due process and to confront witnesses.
Earlier in July US District Judge Jed Rakoff in New York also declared in a ruling that the federal death penalty was unconstitutional, citing evidence that innocent people had been put to death.
Government critical
Both rulings do not affect individual states' laws on the death penalty - by which most death sentences are handed out - but death penalty opponents say the judges' decisions reflect a growing unease in US courts about the way in which the death sentence is dealt out.
The US Government criticised Judge Sessions' decision, noting that the Supreme Court has repeatedly ruled that the death penalty does not violate the US Constitution.
"It is the legislature elected by the American people which determines the proper punishment for federal crimes, not lone members of the judiciary," it said in a statement.
However the US Supreme Court in June this year overturned dozens of death sentences after ruling 7-2 that ruling that juries, not judges, must make death penalty decisions.