By Thomas Escritt
AMSTERDAM (Reuters) - Syria has handed over information about its chemical arsenal to a U.N.-backed weapons watchdog, meeting the first deadline of an ambitious disarmament operation that averted the threat of Western air strikes.
The Organisation for the Prohibition of Chemical Weapons (OPCW) said on Saturday it had "received the expected disclosure" from Damascus, 24 hours after saying it had been given a partial document from Syrian authorities.
It said it was reviewing the information, handed over after President Bashar al-Assad agreed to destroy Syria's chemical weapons in the wake of a sarin gas strike in Damascus's suburbs last month - the world's deadliest chemical attack in 25 years.
Washington blamed Assad's forces for the attack, which it said killed more than 1,400 people. Assad blamed rebels battling to overthrow him, saying it made no sense for his forces to use chemical weapons when they were gaining the upper hand and while U.N. chemical inspectors were staying in central Damascus.
The timetable for disarmament was laid down by U.S. Secretary of State John Kerry and Russian Foreign Minister Sergei Lavrov a week ago in Geneva when they set aside sharp differences over Syria to address the chemical weapons issue.
Their plan set a Saturday deadline for Syria to give a full account of the weapons it possesses. Security experts say it has about 1,000 metric tons of mustard gas, VX and sarin - the nerve gas U.N. inspectors found had been used in the August 21 attack.
The U.S. State Department said on Friday, after the OPCW announced Syria's initial declaration, that it was studying the material. "An accurate list is vital to ensure the effective implementation," spokeswoman Marie Harf said.
Once the OPCW executive has voted to follow the Lavrov-Kerry plan in a meeting expected early next week, the Security Council is due to give its endorsement of the arrangements - marking a rare consensus after two years of East-West deadlock over Syria.
However, the two powers are divided over how to ensure compliance with the accord. U.S. President Barack Obama has warned that he is still prepared to attack Syria, even without a U.N. mandate, if Assad reneges on the deal.
Russia, which says it is not clear who was behind the August 21 attack and has a veto in the Security Council, opposes attempts by Western powers to write in an explicit and immediate threat of penalties under what are known as Chapter VII powers.
It wants to discuss ways of forcing Syrian compliance only in the event that Damascus fails to cooperate.
But a senior Russian official suggested on Saturday that if there were clear indications that Assad were not committed to handing over chemical weapons, Moscow may stop supporting him.
"I'm talking theoretically and hypothetically, but if we became sure that Assad is cheating, we could change our position," said Sergei Ivanov, chief of staff for President Vladimir Putin.
Ivanov said it would take two to three months to decide how long it would take to eliminate Syria's chemical weapons, a task that the Kerry-Lavrov agreement aims to complete by mid-2014.
The accord has been welcomed internationally because of its potential to remove a toxic arsenal from Syria's battlefield and possibly revive international efforts to press for a political solution to the civil war.
But it has done nothing in the short term to stem fighting with conventional weapons, which has killed more than 100,000 people, according to the United Nations.
REBEL OFFENSIVE IN ALEPPO
Rebel forces, some of whom accused the West of betrayal when Obama stepped back from air strikes against Assad's forces three weeks ago, seized several villages south of Aleppo on Saturday.
Their offensive was the latest effort to cut Assad's supply lines to Syria's biggest city, preventing reinforcements by road from Damascus to the south.
Video posted on the Internet showed rebels from the Tawhid brigade firing from a tank and a truck-mounted machine gun at army positions near the Sheikh Said suburb south of Aleppo.
Further south, in Hama province, soldiers and pro-Assad militiamen killed at least 15 people, including a woman and two children, in the Sunni Muslim village of Sheikh Hadid, the Syrian Observatory for Human Rights said.
The British-based group, which monitors violence in Syria through a network of activists and medical and security sources, said the killings followed attacks by rebels on military checkpoints in the area over the previous two days.
It said 26 people - 16 soldiers and 10 members of the pro-Assad National Defence Force - were killed when rebels attacked a nearby checkpoint on Thursday. There was also fighting in the village of Jalma, two miles south of Sheikh Hadid, on Friday, it said.
Syria's civil war, which grew out of a 2011 uprising against four decades of Assad family rule, pits mainly Sunni Muslim rebels against a president whose Alawite faith is an offshoot of Shi'ite Islam.
The war has divided the Middle East along sectarian lines, with Shi'ite Iran and Shi'ite fighters from Iraq as well as Lebanon's Hezbollah backing Assad. Sunni Muslim rulers in Turkey and the Gulf support the rebels, who have been joined by Sunni Islamist fighters from across the region.
The opposition Syrian National Coalition rejected an offer by Iran's president Hassan Rouhani to help start talks with the Syrian government, saying Tehran could not mediate while providing political, economic, and military support to Assad.
"If serious, the Iranian government would withdraw its military experts and extremist fighters from Syria before embarking on dialogue proposals," it said in a statement.
(Additional reporting by Dominic Evans in Beirut, Jason Bush in Moscow; Writing by Dominic Evans; Editing by Kevin Liffey and Andrew Heavens)