The death of Saudi Arabia's King Abdullah early Friday has launched an uncertain new era for U.S. officials to negotiate amid the spreading influence of Iran and the ongoing battle to roll back gains made by the Islamic State terror group in Iraq and Syria.

A former U.S. diplomat close to the Saudi royal family told Fox News Thursday that the death of the 90-year-old King, along with this week's collapse of the U.S.-supported government in Yemen, was a "worst-case scenario" because it removed another obstacle to Iran expanding its reach in the region. The former diplomat said that Tehran's influence could now be seen in four Middle Eastern capitals -- Sana'a in Yemen, as well as Baghdad, Damascus, and to a lesser extent, Beirut. 

Abdullah, a Sunni Arab, made one of the main priorities of his rule countering mainly Shiite Iran whenever it tried to make advances in the region. He also backed Sunni factions against Tehran's allies in several countries, but in Lebanon, for example, the policy failed to stop Iranian-backed Hezbollah from gaining the upper hand. And Tehran and Riyadh's colliding ambitions stoked proxy conflicts around the region that enflamed Sunni-Shiite hatreds — most notably and terribly in Syria's civil war, where the two countries backed opposing sides. Those conflicts in turn hiked Sunni militancy that returned to threaten Saudi Arabia.

With the death of Abdullah, decision-making in Riyadh is likely to be more cautious on issues like Iran and Syria, former U.S. diplomat Dennis Ross told the Wall Street Journal. 

Citing Saudi officials, the paper reports that King Abdullah became less fond of the U.S. in the final years of his reign. The king repeatedly pushed Obama to lend stronger backing to the rebels against Syrian President Bashar al-Assad, against whom he bore a personal animus, and was reportedly furious when airstrikes threatened against Damascus by Obama in the summer of 2013 did not come to pass. 

The officials also said that the late king took a dim view of ongoing talks between the U.S. and Iran over the latter nation's nascent nuclear program, seeing it as a sign that Washington was more than willing to work behind its ally's back. 

King Abdullah's death may also open up a bigger power vacuum in Riyadh than first believed. His successor, 79-year-old half-brother Prince Salman, had recently taken on some of the ailing Abdullah's responsibilities. However, the Journal reports that U.S. officials do not consider him to be a strong or healthy ruler in his own right, which raises the possibility that others in the royal family could come to the forefront. 

The Journal reports that one of the first and biggest questions to face the Saudi king is what to do about the ongoing unrest in Yemen, where gains by Shiite Houthi rebels forced the resignation of the country's president and entire government Thursday. 

There is also the question of what to do about the ongoing U.S.-led bombing campaign against the Islamic State, better known as ISIS. The late King Abdullah was so fearful of the group's growing power that he committed Saudi airpower to strike the Sunni insurgency. 

Among the other decisions facing Salman is whether he will continue the country's ongoing strategy of increased levels of oil production. The country produced 9.6 million barrels a day in January, according to Platts, the energy information division of McGraw Hill. That's enough to satisfy 11 percent of global demand, despite a global price drop of nearly 60 percent since June. 

The price of U.S. crude was up 88 cents, or 1.9 percent, to $47.19 a barrel in after-hours trading Thursday. 

Fox News' Catherine Herridge and The Associated Press contributed to this report. 

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