Saudi Arabia says it has detained 520 al-Qaida militants and supporters so far this year on suspicion of terrorist activities.

The Saudi Interior Ministry said Wednesday that some suspects were planning car bomb attacks against a Saudi oil installation and a security target.

Saudi officials say security forces rounded up 701 terrorism suspects in recent months, but later released 181 for lack of evidence. Among the detainees are Africans and Asians.

The Saudi government says the suspects were part of a terrorist plot managed from abroad. They say one detainee was found with a recorded message from al-Qaida's number-two leader, Ayman al-Zawahri.

U.S. State Department spokesman Tom Casey says the arrests are another indication that al-Qaida and other terrorist groups remain a challenge for the Middle East and the world.

Saudi authorities say security forces seized weapons, ammunition and cash from the suspects in raids on desert hideouts.

They say police found the recorded Zawahri message during the arrest of an al-Qaida cell leader in Saudi Arabia's Eastern Province.

Saudi Arabia is the birthplace of al-Qaida leader Osama bin Laden, whose organization has waged a campaign of violence against the U.S.-allied Saudi monarchy since 2003.

But, Saudi Arabia has not seen any major attacks for more than two years as Saudi leaders intensified a crackdown on al-Qaida-linked groups.

Some information for this report was provided by AP and Reuters.