The US and Russian foreign ministers are due to hold their second round of talks in Geneva on how to secure Syria's chemical weapons.US officials said Thursday's meeting was comprehensive and lasted about an hour. Talks may continue beyond Friday.
The BBC's Paul Adams in Geneva says it appears large disagreements still exist between the two governments.
Syria's president has agreed to give up his chemical arsenal, but the rebels say it will not stop the killing.
Free Syrian Army spokesperson Louay Moqdad told the BBC that the government of President Bashar Assad still had plenty of conventional weapons at its disposal and said it was "just trying to buy time" with the help of the Russians.
But Russian President Vladimir Putin called Damascus's decision to join the Chemical Weapons Convention "an important step towards the resolution of the Syrian crisis" and said it showed the "serious intention" of President Assad "to follow this path".
Syrian conditions
UN-Arab League envoy Lakhdar Brahimi - who has been leading efforts to broker a political solution to the crisis - is due to attend Friday's meeting with Mr Kerry and Mr Lavrov.
Our correspondent says Mr Brahimi will want to know whether the talks can develop into a fresh round of negotiations over a wider political settlement.
Sergei Lavrov said solving the chemical weapons issue would make a strike on the Syrian Arab Republic "unnecessary''
On Thursday, Russian Foreign Minister Sergei Lavrov and US Secretary of State John Kerry said they hoped the plan to put Syria's chemical weapons under international control could avoid the threat of US military action.
Russia announced its proposal for dealing with the escalating chemical weapons crisis on Monday, as the US Congress was preparing to debate whether to back President Barack Obama's moves towards military strikes.
The UN has confirmed it has received documents from Syria on joining the Chemical Weapons Convention, a key step in the Russian plan.
President Assad said data on chemical weapons would start to be passed to the UN after 30 days.
Chemical weapons plan timeline
5-6 Sep:Vladimir Putin and Barack Obama discuss idea of placing Syria's chemical weaponsunder international controlon sidelines of G20 summit
9 Sep:Russia's Foreign Minister Sergei Lavrov says he hasurged Syria to hand in chemical weaponsand have them destroyed; Syria welcomes plan
10 Sep:Syria's foreign minister makesfirst public admissionof the regime's chemical weapons stockpile; Syria commits to Russian plan. Mr Obamapostpones Congress voteon military action and says he will give Russian plan a chance
12 Sep:US Secretary of State John Kerry meets Mr Lavrov in Geneva
13 Sep:Second round of Geneva talks to take place
But Mr Kerry rejected this, saying such standard procedures were not relevant when chemical weapons had already been used.
Washington accuses the Syrian government of killing hundreds of people in a chemical attack in the Ghouta area of the capital, Damascus, on 21 August. The government denies the allegation, blaming rebels.
In an interview with Russian TV, President Assad cautioned that Russia's proposal was "not unilateral", adding: "Syria will accept it if America stops military threats and if other countries supplying the rebels with chemical weapons also abide by the agreement."
Syria's envoy to the UN, Bashar Jaafari, later said that "legally speaking", Syria was now a full member of the convention.
'Not a game'
While UN chief Ban Ki-moon welcomed the application, the UN would not immediately confirm that it had been accepted.
Diplomats said it was possible that the application still had missing elements and it could be sent back to Damascus.
Before Thursday's talks, Mr Kerry said the world was watching to see whether the Assad government would honour its commitments to give up its chemical arsenal.
"This is not a game. It has to be real, it has to be comprehensive, it has to be verifiable, it has to be credible, it has to be... implemented in a timely fashion. Finally, there ought to be consequences if it doesn't take place," he said.
On Russia's Rossiya 24, President Assad says the US must "stop supplying terrorists with weapons"
Mr Lavrov said a "solution" to the chemical weapons issue in Syria would make any military strike by the United States unnecessary.
The US and Russia have sent large teams to Geneva that include weapons experts as well as diplomats.
If the talks in Geneva are successful, the US hopes the disarmament process will be agreed in a UN Security Council resolution.
However, Russia regards as unacceptable any resolution backed by military force, or a resolution that blames the Syrian government for chemical attacks.
Moscow has already objected to a draft resolution that would be enforced by Chapter VII of the UN charter, which would in effect sanction the use of force if Syria failed in its obligations.
Russia, supported by China, has blocked three previous draft resolutions condemning the Assad government.
More than 100,000 people have died since the uprising against President Assad began in 2011.
By BBC
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