Saudi Arabia angrily rejected a UN Security Council seat on Friday, accusing the UN body of "double standards" over the Syria war and other trouble spots in an unprecedented diplomatic assault.
The Saudi move sparked disarray at the Security Council where it only won the seat on Thursday at a UN General Assembly election.
UN leader Ban Ki-moon said Saudi Arabia did not immediately send notification of its decision to reject the term, due to start on January 1.
Diplomats said it could be possible to persuade the Saudi government to reverse the decision.
"Work mechanisms and double-standards on the Security Council prevent it from carrying out its duties and assuming its responsibilities in keeping world peace," the Saudi Foreign Ministry said in a statement.
"Therefore, Saudi Arabia... has no other option but to turn down Security Council membership until it is reformed and given the means to accomplish its duties and assume its responsibilities in preserving the world's peace and security," it added.
The government said "allowing the ruling regime in Syria to kill and burn its people" with chemical weapons is "irrefutable evidence and proof of the inability of the Security Council to carry out its duties and responsibilities".
Saudi Arabia... has no other option but to turn down Security Council membership until it is reformed and given the means to accomplish its duties and assume its responsibilities in preserving the world's peace and security.
Saudi Arabia was one of five nations elected by the UN General Assembly on Thursday to start a two-year term on the Security Council. The others were Chad, Chile, Lithuania and Nigeria. All had stood unopposed.
No country has ever won a council seat and then refused to take it up.
Saudi Arabia's UN ambassador, Abdullah l-Mouallimi, gave several press interviews hailing the election.
But the celebrations had barely finished when the Saudi Foreign Ministry announced the withdrawal.
Russia boycotted council meetings in 1950 in a dispute over who represents China. The council held meetings without them.
In 1980, Cuba and Colombia failed to get a required majority in repeat General Assembly votes. The Council met with 14 members for two weeks until Mexico was finally elected.
If Saudi Arabia maintains the threat, the Asia-Pacific group of nations would have to propose a new candidate for the General Assembly to vote on.
Several envoys said that efforts would be made to persuade Saudi Arabia to take up its seat.
Russia criticised the Saudis' "strange" decision, but the kingdom got a more understanding reaction from Western nations.
The Russian Foreign Ministry sharply criticised Saudi Arabia's "strange" argument on the council's record on Syria.
Russia and Saudi Arabia have a traditionally testy relationship, made worse by Russia's support for Syrian President Bashar al-Assad and Saudi's for opposition rebels.
"The kingdom's arguments arouse bewilderment, and the criticism of the UN Security Council in the context of the Syrian conflict is particularly strange," the ministry said.
However, France said several countries share Saudi Arabia's frustration.
"We think that Saudi Arabia would have brought a very positive contribution to the Security Council, but we do also understand the frustration of Saudi Arabia," France's UN ambassador Gerard Araud told reporters.
Jen Psaki, a spokeswoman for the US State Department, shrugged off the Saudi move, calling it "a decision they have to make".
"I understand different countries will have different responses, but we'll continue to work with them on issues that we share of mutual concern," she said.