It's been known for more than a year that the #1 target of Islamic State in the Middle East is Saudi Arabia. What hasn't been known is the extent of their efforts to undermine the world's top oil exporting country.

The Kingdom revealed yesterday that more than 400 citizens and foreign nationals have been arrested in recent months and they have thwarted several terrorist attacks.

Reuters:

The announcement came after a car bomb exploded at a checkpoint near the kingdom's highest security prison on Thursday, killing the driver and wounding two security officials in an attack claimed by Islamic State.

A string of deadly attacks carried out by followers of the ultra-hardline militant group based in Iraq and Syria has fueled concerns about a growing threat of militancy in the world's top oil exporter.

"The number arrested to date is 431, most of them citizens, in addition to participants from other nationalities ... six successive suicide operations which targeted mosques in the Eastern province on every Friday timed with assassinations of security men were thwarted," the ministry statement posted on the official news agency SPA said.

"Terrorist plots to target a diplomatic mission, security and government facilities in Sharurah province and the assassination of security men were thwarted," it said.

The ministry did not elaborate on when the men were detained, but previous announcements that scores of suspects have been arrested suggest it was over the course of months.

Their alleged offences cited by the ministry ranged from smuggling explosives, surveying potential attack sites, providing transport and material support to bombers, smuggling in explosives from abroad and manufacturing suicide vests.

Islamic State has called on supporters to carry out attacks in the kingdom and killed 25 people in two suicide bombings at Shi'ite Muslim mosques in the country's east in May.

A Saudi man, reportedly aided by several other men from the kingdom, blew himself up in a Shi'ite mosque and killed 27 worshippers in June.

The group says its priority target is the Arabian peninsula and in particular Saudi Arabia, home of Islam's holiest places, from where it plans to expel Shi'ite Muslims.

The interior ministry said the suspects arrested in the kingdom were carrying out "schemes directed from trouble spots abroad and are aimed at inciting sectarian strife and chaos."

The hundreds of IS operatives arrested would indicate a massive effort by Islamic State to destabilize the Kingdom. But IS is hampered by the nature of the opposition in Saudi Arabia.

Unlike Iran and Syria where the armed opposition is largely Sunni Muslim, it's the minority Shias in the Kingdom who oppose the monarchy, while a smaller number of  radical Sunnis wish to overthrow the royal family for their lavish lifestyle and western ways. Neither group is very effective, as they are watched closely by the security services.

But Islamic State has introduced a new and dangerous element into the Kingdom and, as recent history has shown, can cause a lot of trouble. So far, they haven't been able to shake the foundations of the royal family or Saudi society. That may change with time and a few more mass casualty terrorist attacks.