WASHINGTON — Saudi Arabia stunned the United Nations and even some of its own diplomats on 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 it appeared to be unprecedented.

The Saudi Foreign Ministry released a statement rejecting the seat just hours after the kingdom’s own diplomats — both at the United Nations and in Riyadh, the Saudi capital — 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 came 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 al-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 Obama decided against airstrikes on Syria’s military in September in favor of a Russian-proposed agreement to secure Syria’s chemical weapons.