Riyadh (AFP) - Gunmen fired at two US citizens Friday on a road in oil-rich Eastern Province, leaving one wounded, in the fourth anti-Western attack in Saudi Arabia in as many months, police said.

"At 2:00 pm today... a car carrying two American nationals... came under fire from an unknown source, resulting in one of them being wounded and hospitalised," said a police spokesman, quoted by the official SPA news agency.

He was "in stable condition."

The two Americans were travelling on a road in Shiite-populated Al-Ihsaa governorate of eastern Saudi Arabia, he said.

A resident of Al-Ihsaa told AFP that police blocked off a part of the city where a National Guard facility and a Saudi Aramco hospital and training school are located.

No other details were immediately available.

US embassy spokesman Johann Schmonsees, contacted by AFP, said: "We are aware of the reports and are looking into them."

It was the fourth attack since October on Westerners in the kingdom, which in September joined a US-led coalition against the Islamic State (IS) jihadist group in Syria and Iraq.

Last month, Saudi authorities arrested three IS supporters for allegedly shooting and wounding a Dane in his car in Riyadh.

The interior ministry said authorities arrested the suspected gunmen, driver and videographer, who wore masks during the shooting which was filmed and posted online by IS.

The suspects, all Saudis, had "trained for two weeks" before the attack, authorities said.

Also in December, an assailant stabbed and wounded a Canadian as he shopped at a mall in Dhahran on the Gulf coast.

- Lone-wolf attacks -

Canada and Denmark are also taking part in the US-led air campaign against IS.

And in October, a Saudi-American fired from his job with a US defence contractor shot dead an American former colleague and wounded another in Riyadh.

In November, Saudi Arabia blamed IS-linked suspects for the killing of seven Shiites, including children, in Eastern Province.

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In a video posted on Tuesday, Al-Qaeda's Yemen branch (AQAP) called for lone-wolf attacks against the United States and the West, with one of the group's ideologues, Nasser bin Ali al-Ansi, urging supporters to carry out "individual jihad."

A recording was released in November attributed to Abu Mohammed al-Adnani, the IS spokesman, that urged Muslims to attack Westerners by any means, even if only to "spit on their faces".

The video on the Dane being attacked came with an audio recording, purportedly of IS leader Abu Bakr al-Baghdadi, warning that Riyadh will see "no more security or rest".

And the SITE Intelligence Group monitoring service said Tuesday that IS has released a video threatening to invade the Arabian Peninsula soon, "Allah permitting".

Last year, Saudi authorities arrested a man wanted for a number of violent crimes, including shooting at the car of two German diplomats in January 2013.

The Germans escaped unharmed in the incident in the Shiite village of Awamiya in Qatif governorate of Eastern Province.

Awamiya has been a focus of clashes between security forces and protesters from Saudi Arabia's Shiite Muslim minority.

Several Westerners in Saudi Arabia were killed in a wave of Al-Qaeda violence between 2003 and 2007.

The kingdom is home to Islam's holiest sites and practises a strict version of Sunni Islam.