Syria on Thursday sought membership of the global convention banning chemical weapons, in a move that could help head off a western military strike.
Damascus said it now considers itself a full member of the convention. While UN leader Ban Ki-moon welcomed the application, the United Nations would not immediately confirm it had been accepted.
Diplomats, speaking on condition of anonymity, said it was possible there were missing elements and the application could be sent back to President Bashar al-Assad.
"The Syrian letter is currently being studied and until that is completed we don't have anything to say about its status as an instrument of accession," UN spokesman Farhan Haq told AFP.
The United States, which is in talks with Russia on a plan to put Syria's chemical weapons under international control, also demanded "immediate actions" by Syria to surrender its stockpile.
Syria had been one of seven UN members that have refused to join the 1993 convention banning the production and stockpiling of chemical arms.
Assad's government announced it would sign up as it faced a growing threat of a US military strike and Russia proposed a plan to put his country's chemical arsenal under international control.
The United States and other western nations accuse Assad forces of launching a sarin gas attack on August 21 near Damascus in which hundreds died. Washington has threatened a punitive military strike. The government blamed opposition rebels for the attack.
UN spokesman Haq said it could take a few days to complete the accession process.
But Syria's UN ambassador Bashar Jaafari said his government now considered itself a full "legal" member of the convention.
"With this, the chapter of the so-called chemical weapons should be ended," Jaafari told reporters.
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"The chemical weapons in Syria are a mere deterrence against the Israeli nuclear arsenal," he added.
Under the convention, Syria would have 30 days after becoming a member to declare all of its chemical weapons.
Ban announced that Syria had told him that Assad has signed a decree providing for accession to the convention.
"In their letter, the Syrian authorities have expressed their commitment to observe the obligations entailed by the convention even before its entry into force for Syria," said UN spokesman Haq.
"The secretary general welcomes this development," Haq said. Ban added that he hoped it would encourage a "speedy agreement" at Geneva talks between US Secretary of State John Kerry and Russia's Foreign Minister Sergei Lavrov on putting Syrian chemical weapons under international control.
The United States said Syria must immediately follow up the application however.
"This long overdue step does not address the pressing and immediate need for a mechanism to identify, verify, secure, and ultimately destroy Assad's chemical weapons stockpile so they can never again be used," said Erin Pelton, spokeswoman for the US mission at the United Nations.
"For any statement of this kind to be credible, Syria needs to take immediate actions to disclose, surrender, and eliminate its chemical weapons stockpile under international monitoring and verification. Statements without action are wholly insufficient for a country that has had a secret, enormous chemical weapons program for decades."
Pelton said the international community must insist on "consequences for any Syrian noncompliance."
Jaafari said he expected a UN report on the August 21 attack -- which his government blames on opposition rebels -- to be handed to Ban early next week.
"We have nothing to hide," the ambassador said, while adding however that Syria does not want "any partial report, any politicized report, any manipulated report."