Syria became a full member of the global anti-chemical weapons treaty on Thursday, the country's UN envoy said, a move that the government of Syrian President Bashar Assad had promised as part of a deal to avoid U.S. air strikes.

"Legally speaking Syria has become, starting today, a full member of the (chemical weapons) convention," Syrian UN Ambassador Bashar Ja'afari told reporters in New York after submitting relevant documents to the United Nations.

Syria applied on Thursday to sign up to the global ban on chemical weapons, as Kerry and Russian Foreign Minister Sergei Lavrov discussed a Russian plan under which Damascus would give up its arsenal of poison gas.

The United Nations said it had received Syria's application to join the Chemical Weapons Convention, shortly after Assad promised to deliver it within days. Washington immediately warned Syria against stalling tactics to avoid military strikes.

As he began talks in Geneva with Lavrov, Kerry said force might still be needed against Syria if diplomacy over Assad's chemical weapons stockpile fails.

"President Obama has made clear that should diplomacy fail force might be necessary to deter and degrade Assad's capacity to deliver these weapons," Kerry said.

Meanwhile, Reuters reports growing doubts about the accuracy of the figures provided by the White House for fatalities from the August 21 gas attack in Damascus. The government put the total death toll at 1,429, including 426 children, but  Congressional sources said that administration officials had indicated in private that some deaths might have been caused by the conventional bombing that followed the gas attack.

"Expectations are high," Kerry said in Geneva before beginning his talks with Lavrov. "They are high for the United States perhaps even more sides for Russia to deliver on the promise of this moment. This is not a game and I said that to my friend Sergei when we talked about it initially," Kerry said.

"It has to be real. It has to be comprehensive. It has to be verifiable. It has to be credible. It has to be timely and implemented in a timely fashion, and finally there ought to be consequences if it doesn't take place."

Obama, whose attention has been consumed by Syria since he threatened military strikes to punish Assad's government for a poison gas attack that killed hundreds of people in Damascus suburbs two weeks ago, said he was now turning to domestic priorities while backing Kerry's efforts.

This week's eleventh-hour Russian initiative interrupted a Western march to war, persuading Obama to put strikes on hold.

Syria, which denies it was behind that attack, has agreed to Moscow's proposal that it give up its chemical weapons stocks, averting what would have been the first direct Western intervention in a war that has killed more than 100,000 people.

"In the next couple of days, Syria will send a petition to the United Nations and Organization for the Prohibition of Chemical Weapons," Assad said in an interview on Russian TV.

"The petition will contain technical documents required to sign the agreement. After that, work will start that will lead to the signing of the convention prohibiting chemical weapons."

Soon after the interview was broadcast, UN spokesman Farhan Haq told reporters: "In the past few hours we have received a document from the government of Syria that is being translated, which is to be an accession document concerning the Chemical Weapons Convention."

The move would end Syria's status as one of only seven countries outside the international convention that outlaws stockpiling chemical weapons. Other holdouts include regional neighbors Egypt and Israel, as well as North Korea.

The 189-member OPCW is the international organization responsible for implementing the treaty, which documents poison gas stockpiles and oversees their destruction.

Damascus is already bound by the separate Geneva Accords that have banned the use of chemical weapons in warfare for nearly a century, but before this week it had never been required to disclose whether it possessed them. Western states believe it has vast stockpiles of poison gas, including the nerve agents suspected of being used in the Aug. 21 attacks.

A version of the Russian plan that leaked to the newspaper Kommersant described four stages: Syria would join the world body that enforces a chemical weapons ban, declare production and storage sites, invite inspectors, and then decide with the inspectors how and by whom stockpiles would be destroyed.

Blueprint 

U.S. officials said they hoped Kerry and Lavrov could agree on a blueprint with the main points to be adopted in a UN Security Council resolution. An initial French draft calls for an ultimatum to Assad's government to give up its chemical arsenal or face punitive measures.

The Russian initiative offers Obama a way out of a threat to use force, which is deeply unpopular among Americans after 12 years of war in Iraq and Afghanistan. Obama had asked Congress for authorization for strikes and faced a tough fight persuading skeptical lawmakers of the case. That vote is now on hold.

He told a cabinet meeting on Thursday he was now focusing on domestic priorities, while he was hopeful that Kerry's talks with Lavrov would "yield a concrete result".

The sudden pull-back from the brink is a blow for rebels who have listened to Obama and other Western leaders declare in strong terms for two years that Assad must be removed from power while wavering over whether to use force to push him out.

Rebels have long pleaded with the West for advanced weapons to counter Assad's firepower. Obama promised unspecified military aid in June; since then, Washington has trained rebel units but has not delivered arms.

Salim Idriss, the head of the main rebel Free Syrian Army, told U.S. National Public Radio his forces had been poised to launch attacks coordinated with U.S. missile strikes.

"We were and are still waiting for these strikes," he said.

Kerry is accompanied by a large retinue of experts in anticipation of detailed talks on how to turn the Russian offer into a concrete plan along the lines of disarmament accords between Washington and Moscow since the days of the Cold War.

The U.S. delegation will present the Russians with U.S. spy services' assessment of Syria's chemical arms infrastructure, said the U.S. official travelling with Kerry.

Destroying chemical weapons in a war zone will be hard, the official added: "It is doable, but difficult and complicated."