UPDATE 5-Saudi Arabia says arrests 520 terrorism suspects

(Adds details about nationalities, U.S. comment)

By Andrew Hammond

RIYADH, June 25 (Reuters) - Saudi Arabia has detained 520 suspected al Qaeda-linked militants since January, accusing some of them of planning car bomb attacks against an oil installation in the kingdom, the Interior Ministry said on Wednesday.

A ministry statement read out on Saudi television said the detainees were part of a wider plot managed from abroad and involving militant groups seized last year.

Among the detainees were some Asian and African nationals.

Some planned to use car bombs to attack an oil installation and a security target in coordination with al Qaeda second-in-command Ayman al-Zawahri, who had planned to send fighters from Iraq, Afghanistan and North Africa to help them, the ministry statement said.

"Security forces managed to arrest one cell in the Eastern Province led by African residents ... their concern was to get close to people working in the oil sector in order to find work in oil installations," it said.

There were 40 Mauritanians among those held in the province, as well as Afghans, Iraqis and Yemenis, a security source said.

The kingdom, which has faced a campaign of violence by al Qaeda-linked militants since 2003, arrested hundreds of suspects in 2007 but because of a tough security crackdown has not been hit by any major attacks for over two years.