The two American nationals that were targeted by two unnamed gunmen in the Saudi capital of Riyadh were defence contractors for Vinnell Arabia, which works closely with the Saudi National Guard.

One US citizen was shot dead while another was "moderately" wounded after their car, a Chevrolet Tahoe, was attacked by two unknown gunmen. One of the assailants, who was arrested after he was injured in a shootout with the police, was reported to be a Saudi man in his mid 20s, born in the US.

Vinnell Arabia employees have been attacked in the Gulf kingdom before, most notably the May 2003 truck bombing at its compound in Riyadh that killed 10 Americans and 3 Filipino workers. The defence contractor supports Saudi National Guard military programs in Riyadh and the Americans were shot about near to its facility in the city. The site is also near the Saudi National Guard headquarters.

Officials in Riyadh issued a security message to other US citizens after the attack.

The attack happened at 2:49pm at a petrol station, east of the Saudi capital city of Riyadh. Police have confirmed that one of the gunman has been arrested after he was injured in a shootout with the police.

According to a witness, the second American national was reportedly chased into the petrol station shop. The arrested man is believed to be a Saudi man in his mid 20s, born in the USA.

A third American national witnessed the shooting from a separate vehicle. It was not immediately clear if the attack was related to terrorism.

Saudi Arabia is one of the five Arab countries taking part in US-led airstrikes against the Islamic State (also known as Isis) in Iraq and Syria.

On the latest issue of its official magazine, Isis called on jihadi sympathisers across the world to attack westerners "wherever they can be found."

"At this point of the crusade against the Islamic State, it is very important that attacks take place in every country that has entered into the alliance against the Islamic State, especially the US, [the] UK, France, Australia and Germany," the article read.

"Every Muslim should get out of his house, find a crusader and kill him ... And the Islamic State will remain until its banner flies over Rome."