* U.N. report confirms sarin used in Aug. 21 attack
* Allies seek strong U.N. resolution
* Russia says Europeans trying to reinterpret deal
By Warren Strobel and Louis Charbonneau
PARIS/UNITED NATIONS, Sept 16 (Reuters) - The United States, Britain and France warned President Bashar al-Assad on Monday that there would be consequences if he failed to hand over Syria's chemical weapons, and said a U.N. report on the Aug. 21 sarin gas attack left little doubt that Assad's forces were to blame.
As expected, a report by U.N. chemical weapons experts did not say who launched the attack on the rebel-held Damascus suburb of Ghouta, which prompted the threat of Western military action. But it did give details of the type of gas and the munitions used, which some experts said indicated government forces were responsible.
After a meeting of their foreign ministers in Paris, the three Western permanent members of the United Nations Security Council said they would seek a strong U.N. resolution setting binding deadlines for removing Syria's chemical weapons, French President Francois Hollande's office said.
This followed a weekend deal negotiated by Russia and the United States on eliminating the arms.
Russia cautioned against imposing tough penalties on the Syrian leader, who is Moscow's close ally. Russia and Syria say that opposition forces carried out the chemical weapons attack.
In Syria, where rebels fear the U.S.-Russia deal gave Assad license to continue his campaign using conventional weapons, fighting was reported on several fronts. Turkey said its warplanes shot down a Syrian helicopter after it violated Turkish airspace.
The U.S.-Russia deal reached in Geneva put off the immediate threat of U.S. air strikes to punish Assad for the Aug. 21 attack, and Russian Foreign Minister Sergei Lavrov stressed at the time that it did not include any automatic use of force in the event of Syria's failure to comply.
U.S. President Barack Obama has said force remains an option if Assad reneges.
The U.N. report confirmed "unequivocally and objectively" that chemical weapons were used, according to U.N. Secretary General Ban Ki-moon.
"This is a war crime," Ban told the Security Council. "The international community has a responsibility to hold the perpetrators accountable and to ensure that chemical weapons never re-emerge as an instrument of warfare."
Washington says the attack killed more than 1,400 people, including some 400 children
Washington's ambassador to the United Nations, Samantha Power, said the U.N. report made clear that only the Syrian government could have carried it out. British and French officials echoed her comments.
Russian U.N. envoy Vitaly Churkin said there was no scientific proof government forces were responsible for the sarin attack. "We need to not jump to any conclusions," he said.
Syria's U.N. ambassador did not immediately respond to request for comment.
The report confirmed that sarin gas was used. The investigators studied five impact sites and were able to determine the likely trajectory of the projectiles at two sites - Moadamiyah and Ein Tarma.
Eliot Higgins, who blogs under the name of Brown Moses and has been tracking videos of weapons used in the Syria conflict, wrote that he has not seen the opposition using the munitions identified in the report: a variant of the M14 artillery rocket and a 330 mm caliber artillery rocket.
Rebels have seized all kinds of weapons from military depots across the country in the 2 1/2-year civil war.
But Amy Smithson, a chemical weapons expert at the Monterey Institute in California, said the attack bore "so many hallmarks of a military trained in chemical warfare doctrine" and not an untrained force.
"Multiple sites, simultaneously targeted. The early morning hours of the attack are when winds are at their lowest and temperatures at their coolest - the very conditions conducive to having toxic gas stay on the target," she told Reuters.
"The Assad government has been in the business of chemical weapons since the 1970s. They are trained in military doctrine. They also have chemical delivery systems that the rebels don't," she said.
U.S. Secretary of State John Kerry told a news conference in Paris that the three powers agreed with Russia that Assad must suffer consequences if he fails to comply with U.N. demands.
"If Assad fails in time to abide by the terms of this framework, make no mistake, we are all agreed - and that includes Russia - that there will be consequences," Kerry said.
The accord offered the Syrian leader "no lifeline" and he had "lost all legitimacy", Kerry added.
After Hollande met Kerry and British Foreign Secretary William Hague and their French counterpart Laurent Fabius, an aide to Hollande said: "The idea is to stick to a firm line".
"They've agreed to seek a strong and robust resolution that sets precise and binding deadlines with a calendar," said the official, who spoke on condition of anonymity.
Russia accused the Europeans of trying to reinterpret the agreement.
In Moscow, Lavrov said any rush to draw up a resolution threatening to punish Syria in the event of non-compliance showed a "lack of understanding" of the deal.
"Our (European) partners want to again unilaterally review what we've agreed on with the Americans. That's not how you do business, and I'm sure that despite these statements that are coming from European capitals, the Americans will, as proper negotiators, strictly stick to what has been agreed on," he said.
Lavrov said it may be time to consider efforts to force the Syrian opposition to attend an international peace conference instead of just urging them to do so. The rebels have said they will not attend talks if the Syrian president is there.
Syria's government at the weekend hailed as a "victory" the Russian-brokered deal. Rebels who have been fighting Assad's forces since 2011 say it benefited their enemy in the civil war.
Assad briefly dispersed his forces to protect them from strikes threatened by the United States in response to the attack.
Opposition voices say the chemical weapons deal effectively gives Assad permission to carry on with the conventional war, in which more than 100,000 people have died, according to U.N. figures.
Fighting between rebels and government forces ground on from the outskirts of Damascus in the southwest to the central Hama province to Deir al-Zor in the east.
Turkey's Deputy Prime Minister Bulent Arinc said Turkish warplanes shot down a Syrian helicopter after it violated Turkish airspace.
The Syrian Observatory for Human Rights, an anti-Assad monitoring group based in Britain, said government warplanes also hit targets in the Sbeneh area south of Damascus and in the eastern Deir al-Zor province.
The Syrian government has told the United Nations it will adhere to a treaty banning chemical weapons. The U.S.-Russian framework agreement calls for the United Nations to enforce the removal of existing stockpiles by the middle of next year.
Assad has less than a week to begin complying with the deal by handing over a full account of his chemical arsenal. He must allow U.N.-backed inspectors from the Hague-based Organisation for the Prohibition of Chemical Weapons (OPCW) to complete their initial on-site checks by November.
Experts say the removal of up to 1,000 tonnes of chemical agents will be highly problematic in the middle of Syria's civil war, although they assume that the dozens of chemical weapons sites remain under government control.
"The OPCW just doesn't have the manpower to man such an operation like this, so they would bring in other experts," former OPCW official Dieter Rothbacher told Reuters. He said even in normal circumstances it would take 15 to 20 inspectors several months to make an inventory and verify Syria's stockpile.
The U.N. Commission of Inquiry on Syria said on Monday it was investigating 14 alleged attacks with chemical weapons or chemical agents in Syria over the last two years.
U.N. human rights investigators also said hard-line Syrian rebels and foreign fighters invoking jihad, or holy war, had stepped up killings, executions and other abuses in the north since July.