The United States and Russia began high-stakes talks on Thursday on Moscow's plan for Syria to surrender its chemical weapons as Damascus formally applied to join a global poison gas ban, but Secretary of State John Kerry underscored that US military force may still be necessary if diplomacy fails.

"This is not a game," Kerry said in an appearance with Russian Foreign Minister Sergei Lavrov after opening talks in Geneva aimed at fleshing out Russia's plan to secure and dispose of Syria's stockpiles of chemical arms.

The talks were part of a diplomatic push that prompted President Barack Obama to put on hold plans for US air strikes in response to a chemical weapons attack on civilians near Damascus on August 21.

The United States and its allies say Syrian President Bashar al-Assad's forces carried out the attack with sarin nerve gas, killing more than 1,400 people, including 400 children. Russia and Assad blame rebel forces.

The United Nations said it received a document from Syria on joining the global anti-chemical weapons treaty, a move Assad promised as part of a deal to avoid US air strikes.

The move would end Syria's status as one of only seven nations outside the 1997 international convention that outlaws stockpiling chemical weapons. Other holdouts include neighbours Egypt and Israel, as well as North Korea.

The United States immediately warned Syria against stalling tactics to avoid military strikes. Assad told Russian state television in an interview broadcast on Thursday he would finalise plans to abandon his chemical arsenal only when the United States stops threatening to attack him.

Kerry expressed some optimism about the talks in Geneva — expected to last two days — saying, "We do believe there is a way to get this done" and that the United States was "grateful" for ideas put forward by Russia to resolve the crisis.

But he and Lavrov differed sharply on US military threats.

"We proceed from the fact that the solution of this problem will make unnecessary any strike on the Syrian Arab Republic," Lavrov said during the appearance with Kerry.

Russian President Vladimir Putin's Russia has been Assad's most powerful backer during the civil war, which has killed more than 100,000 people since 2011, delivering arms and — with China — blocking three UN resolutions meant to pressure Assad.

"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 asserted.

"Only the credible threat of force — and the intervention of President Putin and Russia based on that — has brought the Assad regime to acknowledge for the first time that it even has chemical weapons and an arsenal, and that (it) is now prepared to relinquish it," Kerry added.

Kerry said any agreement must be comprehensive, verifiable, credible and implemented in a "timely" way — "and finally, there ought to be consequences if it doesn't take place." Kerry called a peaceful resolution "clearly preferable" to military action.

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.

‘Legally speaking’

Syria has agreed to a Russian proposal that it give up its chemical weapons stocks, averting what would have been the first direct Western intervention in the civil war.

"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 documents to the United Nations.

Several UN diplomats and a UN 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.

"The chemical weapons in Syria are a mere deterrence against the Israeli nuclear arsenal," Ja'afari said as he waved a document he called a CIA report on Israeli chemical weapons.

"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," Ja'afari added.

Dressed in a white shirt and dark suit and seated in a wood-panelled office, Assad said in his TV interview that Syria opted to cede control of its chemical weapons because of a Russian proposal and not the threat of US military intervention.

Assad said in comments translated into Russian, "When we see the United States really wants stability in our region and stops threatening, striving to attack, and also ceases arms deliveries to terrorists, then we will believe that the necessary processes can be finalised."

Syria is already bound by the separate Geneva accords that have banned the use of chemical weapons in warfare for nearly a century, but it had never been required before this week to disclose whether it possessed them. Western nations believe Syria has vast stockpiles of poison gas.

Assad said Syria would provide an accounting of chemical weapons stocks in 30 days, standard practice under the treaty.

Kerry questioned the offer. "We believe there is nothing standard about this process at this moment because of the way the regime has behaved — not only the existence of these weapons but they have been used," Kerry said. "And the words of the Syrian regime, in our judgment, are simply not enough."

Kerry and Lavrov flew to Geneva with delegations of chemical weapons and nonproliferation experts to begin to hammer out how to identify, secure and neutralise Syria's chemical weapons.

A draft UN Security Council Resolution submitted by France this week demands that Syria declare its chemical weapons holdings within 15 days, and holds out the threat of sanctions or military force if it fails to disarm.

Hospital bombed

While the diplomats met in Switzerland, the war ground on relentlessly in Syria. Activists said Syrian warplanes bombed one of the main hospitals serving rebel-held territory in the north of the country, killing at least 11 civilians including two doctors.

Video footage showed the limp body of a young child being carried out of the hospital by a man. Another boy lay on the floor, blood on his head and dust covering his body.

Assad's opponents were also accused of atrocities. An anti-Assad monitoring group, the Syrian Observatory for Human Rights, said Sunni Muslim Islamist rebels killed 22 members of Assad's Alawite minority sect in a massacre after storming a village east of the central city of Homs.

Dozens of people have also been killed in the past two days of fighting in the oil-producing northeast of the country, where Sunni Arab militants are battling members of the Kurdish minority in a fight among factions opposed to Assad.

US officials criticised Putin over his remarks in an opinion piece in the New York Times.

Putin said it was alarming that US foreign military intervention had become "commonplace". He also rejected Obama's assertions of "American exceptionalism", saying, "It is extremely dangerous to encourage people to see themselves as exceptional, whatever the motivation.

White House spokesman Jay Carney told reporters: "The fact is that Russia offers a stark contrast that demonstrates why America is exceptional. Unlike Russia, the United States stands up for democratic values and human rights in our own country and around the world. And we believe that our global security is advanced when children cannot be gassed to death by a dictator."