Saudi security forces on Monday shot dead five suspected terrorists believed to be involved in a foiled attack on the world's biggest oil processing complex, the Saudi Interior Ministry said. A sixth suspect was arrested.

The shootings came after security forces raided two houses in the Saudi capital of Riyadh that had been under surveillance, said Lt. Gen. Mansour al-Turki, chief spokesman for the ministry. The suspects were killed during a shootout, the ministry said in a statement.

"We think all the men involved had something to do with Abqaiq attempt," al-Turki said, referring to Friday's attempt by suicide bombers to detonate car bombs inside the world's biggest oil stabilization plant.

Earlier Monday, the Saudi-owned Al-Arabiya television quoted Saudi security sources as saying the forces exchanged fire with the suspected terrorists for about two hours. Police said they confiscated weapons and ammunition from the house in the suburb where the suspects had been holed up, according to the report.

The Saudi branch of Al Qaeda claimed responsibility for Friday's attack, the first ever on Saudi Arabia's vital oil infrastructure. The attack was foiled when security guards fired at two vehicles laden with explosives outside the gates to Abiqaiq, which processes about two-thirds of the country's oil before it is exported.

Two guards and two would-be suicide bombers were killed. The Interior Ministry identified the attackers Sunday as Abdullah Abdul-Aziz al-Tweijri and Mohammed Saleh al-Gheith, and said both were on a list of the 15 most-wanted terrorists the kingdom issued in June.

The deaths of the two meant that only four remain at large of the list of 15. Ten have now died or been killed, and one has been arrested.

The Saudi branch of Al Qaeda warned in an Internet statement Saturday that its suicide bombers would strike the oil industry again.

Al Qaeda militants launched a campaign of violence in Saudi Arabia in 2003. The country is the birthplace of Al Qaeda leader Usama bin Laden.

Saudi security forces have largely had Al Qaeda militants on the run for the past year, arresting hundreds of suspects. They killed or captured all but one of the top 26 militants on a most-wanted list issued in December 2003, then issued the second list in June.

Saudi Arabia holds over 260 billion barrels of proven oil reserves, a quarter of the world's total. It currently puts out about 9.5 million barrels per day, or 11 percent of global consumption.