Four people were killed and 18 wounded on Friday when a suicide bomber attacked a mosque in Shia-populated eastern Saudi Arabia, state media said.

The Al-Ekhbariya news channel quoted an interior ministry spokesman as giving the new toll, up from two dead and seven wounded earlier.

Saudi authorities prevented two suicide bombers from entering Imam Rida Mosque in Mahasen neighborhood in Al-Ahsa region during Friday prayers, Saudi state news agency SPA reported, citing a statement by the ministry's spokesman.

“When security men stopped them, one blew himself up at the entrance of the mosque and there was an exchange of fire with the other. He was wounded and arrested wearing an explosive belt,” the spokesman said.

Witness Mohammed al-Nimr told The Associated Press that security forces and ambulances quickly surrounded the mosque. He said worshippers stopped the attacker from detonating a suicide bomb belt.

Al-Nimr is the brother of Sheikh Nimr al-Nimr, who was executed earlier this month by Saudi officials, raising tensions in the area.

In the attack's chaotic aftermath, Saudi police fired assault rifles into the air to drive away an angry mob that surrounded a police car holding the suspected attacker, according to video shot from the scene.

Shia Muslims in Saudi Arabia make up between 10 to 20 percent of the ultra-conservative, Sunni-ruled kingdom's population. The minority group, many of whom live in the country's oil-producing east, previously have been targeted in attacks by the Islamic State in Iraq and the Levant (ISIL), which views Shias as heretics. No group immediately claimed Friday's attack.

Wire services