A special anti-terror court has sentenced three Saudis to six years in jail each for a plot to kill US nationals at the height of a campaign of Al-Qaeda attacks in the kingdom between 2003 and 2006, a witness said.

The court in Riyadh issued its ruling late on Sunday, said a witness at the hearing during which two other men were acquitted.

The three convicted were found guilty of having "supported the actions of the deviant group and planned the assassination of seven Americans in the kingdom," the court ruled, using the official term to designate Al-Qaeda.

It said they also planned to enter Iraq "to carry out terrorist acts".

Saudi courts started in July 2011 to try hundreds of suspects for the wave of Al-Qaeda attacks which rocked the country from 2003 until 2006 when authorities launched a massive campaign of arrests targeting the jihadist network.

Around 3,000 suspects are being tried or due to go on trial, according to Saudi authorities.