Saudi Arabia has executed seven men for an armed robbery, the interior ministry said, despite an appeal for clemency by United Nations human rights investigators.

The seven, some of whom were under 18 when the crime was committed in 2006, had been due to be put to death last week but were granted a stay of execution while authorities reviewed their case.

Human rights activists in Saudi Arabia said they had been executed by firing squad, but the country's official news agency said they had been beheaded. The original sentences called for death by firing squad and crucifixion.

The men, from Asir province in the south of the country, were convicted of robbery and theft.

One of the men told Associated Press earlier this month that he was only 15 when he was arrested as part of a ring that stole jewellery in 2004 and 2005. Nasser al-Qahtani said he was tortured to confess and had no access to lawyers.

"The charges against all seven persons were allegedly fabricated and all seven were convicted following unfair trials," the UN experts said in a statement on Tuesday.

The conservative Islamic kingdom, which applies a strict interpretation of sharia, has faced criticism by western countries for its frequent use of capital punishment and trials that human rights groups say do not meet international standards.