WASHINGTON — Saudi Arabia stunned the United Nations and even some of its own diplomats Friday by rejecting a highly coveted seat on the Security Council, a decision that underscored the depth of Saudi anger over what the monarchy sees as weak and conciliatory Western stances toward Syria and Iran, Saudi Arabia’s regional rival.

The Saudi decision — which could have been made only with King Abdullah’s approval — came a day after it had won a Security Council seat for the first time, and appeared to be unprecedented.

The Saudi Foreign Ministry released a statement rejecting the seat just hours after the kingdom’s diplomats — both at the U.N. and in Riyadh — were celebrating their new seat, the product of two years of work to assemble a crack diplomatic team in New York. Some analysts said the sudden turnabout gave the impression of a self-destructive temper tantrum.

But one Saudi diplomat said the decision had come after weeks of high-level debate about the usefulness of a seat on the Security Council, where Russia and China have repeatedly drawn Saudi anger by blocking all attempts to pressure Syria’s president, Bashar Assad. Abdullah has voiced rising frustration with the continuing violence in Syria, a fellow Muslim-majority nation where one of his wives was born. He is said to have been deeply disappointed when President Barack Obama decided against airstrikes on Syria’s military in September in favor of a Russian-proposed agreement to secure Syria’s chemical weapons.

And Saudi officials made no secret of their fear that a nuclear deal between Iran and the West — the subject of multilateral talks this week in Geneva with another round scheduled for early November — could come at their expense, leaving them more exposed to their greatest regional rival.

The Saudi decision may also reflect a broader debate within the Saudi ruling elite about how to wield influence: The Saudis have long resisted taking a seat on the Security Council, believing it would hamper their discreet diplomatic style.

Still, the sudden about-face came across as a slap to the United Nations and the United States, one of Saudi Arabia’s strongest Western allies. On Thursday evening, the U.S. ambassador to the United Nations, Samantha Power, had issued a statement congratulating the five new nonpermanent members — Chad, Chile, Lithuania, Nigeria and Saudi Arabia. Officials at the U.S. Mission to the United Nations had no immediate comment.

Many experts had assumed that Saudi Arabia’s pursuit of a Security Council seat signaled a new desire to be more public and assertive in its stances toward the Syrian civil war and the Arab-Israeli conflict. The Saudi ambassador to the United Nations, Abdallah al-Mouallimi, was clearly elated after the General Assembly vote Thursday.

“We take this election very seriously as a responsibility to be able to contribute to this very important forum to peace and security of the world,” he told reporters. “Our election today is a reflection of a longstanding policy in support of moderation and in support of resolving disputes by peaceful means.”

The statement Friday struck a far different tone, calling for changes to enhance the Security Council’s contribution to peace. It did not say what those should entail.

“Allowing the ruling regime in Syria to kill and burn its people by the chemical weapons, while the world stands idly, without applying deterrent sanctions against the Damascus regime, is also irrefutable evidence and proof of the inability of the Security Council to carry out its duties and responsibilities,” the statement said.

The statement accused the Security Council of failing to find a “just and lasting solution” to the Israel-Palestine conflict, and of failing to free the Middle East of “all weapons of mass destruction,” an apparent reference to Israel’s presumed nuclear arsenal.

“This is very bad for the image of the country,” said one Saudi political insider, who requested anonymity because the decision was assumed to be by the king, whose judgment is rarely questioned in public. “It’s as if someone woke up in the night and made this decision.”