* U.N. report confirms sarin used in Aug. 21 attack
* Allies seek strong U.N. resolution
* Idea is to take "firm line", says France
* Russia says Europeans trying to reinterpret deal
By Warren Strobel and Alexander Dziadosz
PARIS/BEIRUT, Sept 16 (Reuters) - The United States, France and Britain warned President Bashar al-Assad on Monday that there would be consequences if he fails to stick to a deal under which Syria must give up its chemical weapons, and U.N. experts confirmed sarin gas was used in the Aug. 21 attack in Damascus.
Russia, which negotiated the deal with the United States, cautioned against imposing tough penalties on the Syrian leader, who is Moscow's close ally. In Syria, fighting was reported on several fronts, and Turkey said its warplanes shot down a Syrian helicopter after it violated Turkish airspace.
The three Western permanent members of the United Nations Security Council said they would seek a strong U.N. resolution setting binding deadlines for the removal of Syria's chemical weapons, French President Francois Hollande's office said.
A U.N. report on the Aug. 21 attack 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."
As expected, the report did not say who carried out the attack. Ban said on Friday that Assad "has committed many crimes against humanity," although he did not ascribe blame for this specific incident.
Assad and Moscow have blamed the rebels.
The United States reached a deal at the weekend with Russia that could avert U.S. strikes on Syria as punishment for last month's attack.
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.
Speaking in Moscow, Foreign Minister Sergei 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 agreement reached for Assad to give up his chemical weapons.
"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," Lavrov said.
PEACE TALKS PLAN
He also said it may be time to consider efforts to force the opposition to attend an international peace conference instead of just urging them to do so. So far, the rebels have said they will not attend talks if the Syrian president is there too.
The deal reached in Geneva put off the immediate threat of air strikes, and Lavrov stressed at the time that it did not include any automatic use of force in the event of Syria's failure to comply. But President Barack Obama has said force remains an option if Assad reneges.
Syria's government at the weekend hailed as a "victory" the Russian-brokered deal, which rebels who have been fighting Assad's forces since 2011 say has 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 chemical weapons attack in Damascus, which Washington says killed more than 1,400 people, many of them children.
The report by chief U.N. chemical weapons investigator Ake Sellstrom, which was presented to the U.N. Security Council on Monday, said there was "clear and convincing evidence that surface-to-surface rockets containing the nerve agent sarin were used" in that attack.
Opposition voices say the chemical weapons deal effectively gives Assad permission to carry on with his conventional war, in which so far more than 100,000 people have died, according to U.N. figures.
Fighting between rebels and government forces, which often kills more than 1,000 people a week, 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.
Some rebels earlier reported that a warplane which appeared to be non-Syrian - and which some of the rebels presumed was Turkish - had shot down the helicopter, according to the Syrian Observatory for Human Rights, an anti-Assad monitoring group based in Britain.
The reports contradicted other rebel accounts that it was insurgents who shot down the aircraft, the Observatory said.
The Observatory said the helicopter had gone down in an area between the Jisr al-Shughour and northern Latakia areas, near the border with Turkey's Antakya region.
Government warplanes also hit targets in the Sbeneh area south of Damascus and in the eastern Deir al-Zor province, according to the Observatory, which has a network of sources across Syria.
The rebels have struggled to counter Assad's air power, but Western countries have been wary of supplying them with sophisticated anti-aircraft weapons they fear may end up in the hands of anti-Western Islamist factions.
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.
Assad told Russian state television last week that his cooperation was dependent on an end to threats of war and to U.S. support for rebel fighters. But it seems likely that Moscow can prevail on him to comply, at least initially, with a deal in which it has invested considerable personal prestige.
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. "Moving an entire stockpile is something that has never been done before."
He estimated that even in normal circumstances it would take a team of 15 to 20 inspectors several months to take 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 hardline Syrian rebels and foreign fighters invoking jihad, or holy war, had stepped up killings, executions and other abuses in the north since July.
There were now a number of brigades made up entirely of non-Syrians, underlining how the 2-1/2-year-old conflict has pulled in neighbouring countries and widened sectarian faultlines across the region.
"The point is that these extreme elements have their own agenda and certainly not a democratic agenda that they are seeking to impose," investigator Vitit Muntarbhorn told Reuters.