By Louis Charbonneau and Michelle Nichols
UNITED NATIONS (Reuters) - Syria became a full member of the global anti-chemical weapons treaty on Thursday, the country's U.N. envoy said, a move that the government of Syrian President Bashar al-Assad had promised as part of a Russian plan to avoid U.S. air strikes.
The United States reacted cautiously. Several U.N. diplomats and a U.N. official told Reuters on condition of anonymity that it was not yet clear that Syria had fulfilled all the conditions for legal accession to the treaty.
"I think there are a few more steps they have to take (before Syria is a signatory) but that's why we're studying the document," a U.N. official said.
Syria was one of only seven countries not to have joined the 1997 Chemical Weapons Convention, which commits members to destroying their stockpiles.
"Legally speaking Syria has become, starting today, a full member of the (chemical weapons) convention," Syrian U.N. Ambassador Bashar Ja'afari told reporters in New York after submitting relevant documents to the United Nations.
Erin Pelton, spokeswoman for the U.S. mission to the United Nations, said Syria's declared intention to join the Chemical Weapons Convention was just a first step.
"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," Pelton said. "It also does not include immediate consequences for non-compliance."
"The international community must insist on immediate progress toward destruction of Syria's CW (chemical weapons) program and also make clear that there will be consequences for any Syrian noncompliance," she said.
Pelton added that the United States would "continue our ongoing discussions with the Russians and key international partners to urgently respond to the Assad regime's blatant disregard for international norms."
U.S. Secretary of State John Kerry and Russian Foreign Minister Sergei Lavrov are meeting in Geneva to try to hammer out the details of a Russian plan for Syria to place its chemical arsenal under international control.
Syria agreed to the plan after the United States threatened military strikes to deter it from using chemical weapons after an August 21 gas attack killed hundreds of people.
Assad's government blames the rebels for that attack. Washington blames the government and says the sarin gas used in it killed more than 1,400 people, including many children.
Russia, a staunch Assad ally, has vetoed three U.N. Security Council resolutions that would have condemned Assad's government and threatened it with sanctions. The United States has accused Russia of holding the council hostage on the question of Syria.
According to the Chemical Weapons Convention, a country does not become a full member of the treaty until 30 days after its accession or ratification documents are deposited with the United Nations.
U.N. Secretary-General Ban Ki-moon's press office confirmed receipt of a letter from Syria that said Damascus would immediately begin complying with the terms of the treaty.
"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," it said in a statement.
U.N. spokesman Farhan Haq told reporters earlier on Thursday that the United Nations had received an "accession document" from Syria that was currently being studied and translated.
Ja'fari said Assad signed a legislative decree on Thursday that "declared the Syrian Arab Republic approval to accede to the convention." He said Foreign Minister Walid al-Moualem had written to the Organization for the Prohibition of Chemical Weapons to notify it of Syria's decision to join the convention.
"The chemical weapons in Syria are a mere deterrence against the Israeli nuclear arsenal," Ja'afari said as he waved a document he said was a CIA report on Israel's chemical weapons program.
"It's a deterrent weapon and now the time has come for the Syrian government to join the (convention) as a gesture to show our willingness to be against all weapons of mass destruction," he said.
(Reporting by Louis Charbonneau and Michelle Nichols; Editing by Will Dunham)