Saudi Arabia, a top OPEC player, has arrested more than 700 suspected Islamist militants in the past six months for allegedly plotting attacks on oil industry installations, the interior ministry said Wednesday.

The 701 militants from various countries were arrested in multiple waves, but 181 were later released because there was no proof linking them to the terror network, according to a ministry statement carried by the official Saudi Press Agency. The remaining 520 are still in custody, it added.

Security forces "carried out several operations against followers of the deviant ideology and arrested a total of 701 people of various nationalities," a ministry spokesperson was quoted by the Saudi Press Agency.

References in the statement to "deviant group" and "deviant ideology" are Saudi euphemisms for Al Qaeda and its sympathizers.

Riyadh has in the past reported arrests of large numbers of militants, including those linked to Al Qaeda, but the figure released Wednesday was the highest to date.

Al Qaeda has called for attacks against the Saudi government in the past, criticizing its alliance with the U.S. and hoping to disrupt the flow of oil to the West. The group has also labeled the Saudi government un-Islamic, even though the kingdom follows an austere strain of Islam known as Wahhabism.

State Department spokesman Tom Casey said the reports of the arrests were an indication "a lot more" needed to be done to combat terrorism worldwide.

"It's just another indication that Al Qaeda and the terrorist groups out there remain, and remain a challenge, not only for the United States and for Saudi Arabia but for the broader region and, really, for the world," said Casey.

The ministry said police found money, weapons and ammunition owned by the suspects, who had buried some of it in remote areas. The men from regions that included Africa and Asia were organized into various cells, whose leaders were based outside Saudi Arabia, the statement added.

One of the men was reportedly found with a message from Al Qaeda's No. 2 leader, Ayman al-Zawahri, urging him to raise money and saying the terrorist group would provide militants from Iraq, Afghanistan and North Africa "to target oil installations and fight security forces."

The statement said some of the militants were planning car bomb attacks against an oil installation and a "security" target, but didn't provide further details. In February 2006, two explosives-laden vehicles tried to enter the Abqaiq oil complex, the world's largest oil processing facility, in eastern Saudi Arabia. But guards opened fire and the vehicles exploded without damaging the facility.

The ministry said several of the suspects arrested this year were trying to get jobs inside oil installations in eastern Saudi Arabia, which holds much of the reserves of the world's top crude exporter.

"The cells that have been broken up which were run from abroad were primarily targeting economic targets in the country," which is the world's second-largest oil producer and its largest exporter, the ministry spokesperson told the Saudi Press Agency.

Another man who was arrested was trying to raise money in the western Saudi oil hub of Yanbu, where attackers stormed the offices of a Houston-based oil company in 2004.

Some of the suspected militants also tried to recruit Saudis "by spreading misleading propaganda on the Internet" and undermining government-appointed clerics, said the ministry.

Saudi Arabia, the birthplace of Al Qaeda leader Usama bin Laden, has pursued an aggressive campaign against militants over the past five years, and its security forces have managed to kill or capture most of those on its list of most-wanted Al Qaeda loyalists in the country.

In April 2007, Saudi Arabia announced one of its largest anti-terrorism sweeps that netted 172 Islamic extremists. Those arrests stopped plans to mount air attacks on the kingdom's oil refineries, break militants out of jail and send suicide attackers to kill government officials.

The Associated Press and AFP contributed to this report.