Violence in Saudi Arabia's Shiite-dominated Eastern province, center of the kingdom's oil industry, has intensified the political jitters as the monarchy frantically tries to fend off the Arab Spring, settle a thorny succession problem and confront old rival Iran.

There have been flare-ups involving disgruntled minority Shiites, who number around 2 million, in the past in the overwhelmingly Sunni kingdom.

But Monday's violence involved gunmen on motorcycles and firebombs, an unusual occurrence that could signal trouble yet to come.

The Saudis don't usually publicize such disturbances but the Interior Ministry vowed Tuesday to use "an iron fist" against any further trouble and claimed Monday's clashes were instigated by "a foreign country."

That's a clear reference to Iran, which is locked in a struggle for supremacy in the Persian Gulf region with Saudi Arabia, the world's top oil producer and the birthplace of Islam.

This rivalry swelled into outright confrontation in March when Riyadh sent tanks and troops of the Saudi-led Gulf Cooperation Council into the neighboring Sunni kingdom of Bahrain to crush protests led by the Shiite majority. These were widely deemed to have been instigated by Tehran.

Riyadh fears Bahrain's Shiite unrest will spread to the Eastern Province.

The unprecedented turmoil of the so-called Arab Spring over the last eight months hit Saudi Arabia at a bad time.

The U.S. abandonment of Egyptian President Hosni Mubarak, a longtime ally forced to step down Feb. 11, stirred fears among the Saudi royals that the Americans couldn't be trusted to stand by them.

King Abdallah, believed to be 86, is in poor health after being hospitalized in New York earlier this year for what was reported to be a back injury. His designated successor, Crown Prince Sultan, the longtime defense minister, is slightly younger, but he too is in bad shape. He was treated in the United States for cancer and is so infirm he rarely appears in public and isn't considered fit to rule.

The third in line, the powerful Interior Minister Prince Nayef, is believed to be 76.

The leading clans within the extended royal family -- at least 4,000 princes, though few are contenders for the throne -- are locked in a Byzantine struggle for supremacy.

But the time is clearly close when the crown will have to pass from the sons of King Abdel Aziz, founder of the modern state, who have ruled since his death in 1953 to his grandsons.

In the past, the House of Saud has been able to overcome family feuds to find a compromise when it comes to the succession. But now it's facing uncharted territory.

Other issues also divide the royal family. There's growing concern in Riyadh over the worsening violence in neighboring Yemen, where the Saudis have long called the shots.

Efforts by the Saudis, the key mediator in Yemen's crisis, to end the bloodletting that began in January with street protests demanding the ouster of President Ali Abdullah Saleh have failed.

As Yemen lurches toward all-out civil war, the Saudi leadership is increasingly split over how to deal with Saleh, who refuses to step down. Abdallah favors backing Saleh, who over the years has been the recipient of Saudi largesse, while Nayef wants Saleh gone.

This, U.S. global security consultancy Stratfor observed, "has prolonged the political stalemate in Yemen."

Nayef "has been advocating moves to undercut Saleh's support, and Prince Mayef's faction has financially supported leading opposition members and tribesmen against Saleh loyalists," Stratfor noted.

"King Abdallah's faction, however, has had a much higher tolerance for the Yemeni president and has argued for a much more moderate Saudi policy in managing the Yemeni crisis."

The Saudis are alarmed that a political vacuum in Yemen and the splintering of the country's military forces will allow the al-Qaida in the Arabian Peninsula to unleash a terror campaign against the monarchy as it did in 2003-06.

That failed through a ruthless Saudi counterinsurgency operation. But a new campaign amid the political turmoil of the Arab Spring and the deepening confrontation with Iran could be more successful.

If the trouble in the Eastern Province grows, encouraged and inspired by Tehran, the threat to the Saudi oil fields would be magnified, sparking an international crisis of confidence in the world's largest family business.