Followers of the Islamic State militant group from Saudi Arabia have the oil-rich kingdom in their crosshairs and they may attack while the Saudi throne is being handed down, intelligence experts on the Middle East warned.

Reports said Islamic State militants posted a video announcing plans to seize Saudi Arabia, after the death of its king. A group of Saudi militants urged followers in the kingdom to carry out attacks within, reported SITE Intelligence, a group that keeps tabs on jihadist movements.

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Experts said the warning revealed the Islamic State's target of annexing the Middle East's richest country and take control of Islam's holiest site, Mecca. The University of Cambridge's Toby Matthiesen told a U.S. network that Mecca and Medina are the terrorists' biggest prize. He added that if the group's goal is to rebuild the caliphate, the two sacred cities are vital.

The Saudi kingdom is fighting Islamic State fighters on its border with Iraq and it is trying to quell unrest near its border with Yemen to the south, where several terrorist groups managed to take control of the president's palace. Authorities have been keeping hostile forces at bay as the country made a quick transition after King Abdullah's death on January 23.

The kingdom was quick to announce that the king's successor would be Abdullah's half-brother Salman bin Abdul-Aziz Al Saud, 79.

Islamic State, which controls territory straddling Syria and Iraq, commands thousands of Saudi followers. Newsweek reported that militants carried out a late-night attack on the Saudi border and apparently made it as far as Rafha, a northern town.

The United States counts Saudi Arabia as an important ally in the Middle East. Besides its petroleum wealth, the kingdom has contributed air support in the military campaign against Islamic State. U.S. President Barack Obama cut short his trip to India this week to pay respects to Abdullah and to greet the new king who welcomed him at the airport. 

TagsISIS, King Salman