Gunmen killed at least five people in Saudi Arabia's eastern region, the state news agency SPA reported on Tuesday, in what local residents said was an attack on Shiite Muslim worshippers marking one of their most important religious anniversaries.

Whatever its motive, the raid late on Monday in al-Ahsa district may test the already strained relations between Sunnis and Shiites across the Middle East as it coincides with the annual Ashoura commemoration of Shiite Islam.

An Interior Ministry spokesman said six people had been arrested in connection with the attack in al-Dalwah village, the report said.

"As a group of citizens was leaving a building ... three masked men opened fire at them with machine guns and pistols", the spokesman was quoted as saying. The incident is still under investigation.

Al-Ahsa is one of the main centers of minority Shiite Muslims in Sunni-ruled Saudi Arabia. The Shiites are celebrating Ashoura, a holy day with public ceremonies and processions commemorating the death of the Prophet Muhammad's grandson, Hussein, at the battle of Kerbala in AD 680.

A local man said the victims were mostly young men standing at the entrance of the local gathering place, known as Huseiniya, where the commemorative ceremony was taking place.

"It seems the criminals were in a hurry and opened fire on youngsters at the entrance and fled," Ali al-Bahrani told Reuters by telephone.

He said there were reports that Saudi security forces had found a vehicle, apparently used by the attackers, with automatic weapons inside and arrested one person.

"This seems to be the work of criminals and terrorists trying to mix cards, but security authorities seem determined to strike with an iron fist," he told Reuters.

The Saudi-owned al-Arabiyah satellite news channel said six suspects linked to the attack have been detained.

High alert

More than 1 million Shiite Muslims gathered at shrines and mosques across Iraq on Tuesday for Ashoura, with Iraqi security forces on alert for any repeat of the kind of attacks that have inflicted mass casualties during past pilgrimages.

The presence in the country of hard-line Islamic State militants, who swept through the northern region earlier this year, raises the possibility of wider bloodshed as crowds swell.