* U.N. inspectors sit in Damascus, despite calls for access to scene
* Divided U.N. Security Council calls for "clarity" over alleged attack
* Syrian authorities dismiss report as untrue, no comment on access
* Assad ally Russia says looks like rebel "provocation"
* France says world should respond with force if chemicals used (Recasts, edits, adds international reactions)
By Erika Solomon and Khaled Yacoub Oweis
BEIRUT/AMMAN, Aug 22 (Reuters) - Western powers demanded Syria give U.N. chemical weapons experts immediate access on Thursday to rebel-held Damascus suburbs where poison gas appears to have killed hundreds just a few miles from the U.N. team's hotel.
There was no sign, however, that they would soon be taking take samples at the scene of horrors that have drawn comparison with the gassing of thousands of Iraqi Kurds at Halabja in 1988.
President Bashar al-Assad's opponents gave death tolls from 500 to well over 1,000 and said more bodies were being found in the wake of Wednesday's mysterious pre-dawn killer fumes, which the Syrian government insists were not of its making.
Talk, notably from France and Britain, of a forceful foreign response remains unlikely to be translated into rapid, concerted action given Russian opposition and deep caution in Washington.
French Foreign Minister Laurent Fabius said world powers must respond with force if allegations that Syria's government was responsible for the deadliest chemical attack on civilians in a quarter-century prove true; but even Fabius stressed there was no question of sending in troops on the ground.
Britain, too, said no option should be ruled out "that might save innocent lives in Syria". But European forces can do little without U.S. help and Washington shows little appetite for war.
Syrian authorities have called allegations against their forces "illogical and fabricated", pointing to the timing of the attack and their previous assertions that, if they possessed chemical weapons, they would never use them against Syrians.
Key Assad ally Moscow pointed the finger at a "provocation" by rebels keen to draw in Western military assistance.
After months of negotiating with Assad's government to let inspectors into Syria, a U.N. team arrived in Damascus four days ago. Their task is to check on the presence, but not the sources, of chemical weapons that are alleged to have been released in three specific, small incidents several months ago.
They have no mandate beyond that. A divided U.N. Security Council, meeting in emergency session on Wednesday, failed to endorse a Western push for an immediate inspection of the sites near Damascus. It called only for "clarity" on the incident.
Syria's government offered no immediate public response to calls on Thursday for the U.N. team to have access to the area.
"WE'RE BEING EXTERMINATED"
Many rebels and activists in the opposition area say they have lost interest in promises of U.N. investigations or in help from abroad: "We are 7 kms away, just a 5-minute car ride from were they are staying," said activist Bara Abdelrahman.
"We're being exterminated with poison gas while they drink their coffee and sit inside their hotels."
Qassem Saadeddine, a commander and spokesman for the rebels' Supreme Military Council, said the group was still deliberating on how or if it should respond to the attack.
"People are growing desperate as they watch another round of political statements and U.N. meetings without any hope of action," he told Reuters.
"We are still studying how the rebels should respond."
Syria's revolt against four decades of Assad family rule has turned into a brutal civil war that has killed more than 100,000 people in two and half years and divided the Middle East along largely sectarian lines. Among world powers the conflict has revived Cold War-era East-West tensions and on the ground the struggle has limped to a poisonous stalemate.
Western powers back the opposition but have been reluctant to fully commit to a revolt increasingly overtaken by Islamists linked to al Qaeda. Yet they have said the large-scale use of widely banned chemical weapons would be a game changer.
"We are asking for this team to go directly, with complete freedom ... to the site of the crimes which took place yesterday," George Sabra, a prominent member of the umbrella opposition's National Coalition, told Reuters.
He said the U.N. Security Council should amend the team's mission to give it a mandate to visit any site.
"We are doubtful," he conceded, however. "Because the mission of these experts is constrained by the Syrian regime, limited to a few areas which it will take them to."
CALL FOR ACTION
In Paris, Fabius called for action if the allegations proved true: "There would have to be reaction with force in Syria from the international community, but there is no question of sending troops on the ground," he told the BFM television network.
If the Security Council could not make a decision, one would have to be taken "in other ways", he said, without elaborating.
But immediate international action is likely to be limited.
European officials speaking on condition of anonymity said that options ranging from air strikes, creating a no-fly zone, or providing heavy weapons to some rebels were all options still on the table - but that there was little prospect of concrete measures without U.S. backing, which still seemed unlikely.
"The American reaction following yesterday's attack was cautious," said one. "And without U.S. firepower there's little we can do."
While France and Britain, the main military powers in the European Union, took a lead in attacking Muammar Gaddafi's forces to help Libya's revolt in 2011, the ultimately successful campaign against an enemy far weaker than Assad's Russian-armed military also relied heavily on U.S. firepower and logistics.
Assad's forces have continued a heavy bombardment of the Ghouta region, which activists say will further hinder U.N. investigators from entering the area.
Washington and European allies say Assad's forces have used small amounts of sarin nerve gas before, hence the current U.N. visit. But Assad's forces have also levelled accusations of chemical weapons use against the rebels, particularly an incident in Khan al-Assal, near Aleppo, where 25 people died.
FOAMING AT MOUTH
A spokesman from the opposition's Syrian National Coalition said bodies were still being found on the outskirts of Damascus.
"We expect the number to grow because we just discovered a neighbourhood in Zamalka where there are houses full of dead people," said Khaled Saleh, speaking in Istanbul.
Fahad Almasri, spokesman for the rebel Free Syria Army in Paris, said its branch in Damascus had documented 1,729 deaths so far - a figure several hundred higher than other opposition groups have given. Thousands more had breathing difficulties.
Opposition activists said men, women and children were killed as they slept. They say several towns in Ghouta were hit with rockets loaded with poison gas before dawn.
Images, including some by freelance photographers supplied to Reuters, showed scores of bodies laid out on floors with no visible signs of injury. Some had foam at the nose and mouth.
The United States said it has no independent confirmation that chemical weapons had been used. U.N. Secretary-General Ban Ki-moon said on Wednesday the head of the inspection team in Damascus was discussing the latest claims with the government.
Syria is one of just a handful of countries that are not parties to the international treaty that bans chemical weapons, and Western nations believe it has caches of undeclared mustard gas, sarin and VX nerve agents.
Weapons experts said it was unclear from evidence so far what precise chemicals may have been involved and how they may have been delivered. While opposition groups spoke of rockets carrying gas canisters, analysts abroad said gases, possibly a cocktail of compounds, could have been released in other ways.
While Assad's armed forces are suspected of having such stocks, analysts also noted that rogue units, not under direct orders from the government might choose to use them - as might some opposition group, should it have captured such weapons. (Additional reporting by John Irish in Paris, Anthony Deutsch in Amsterdam, Niklas Pollard in Stockholm, Thomas Grove in Moscow and Louis Charbonneau and Michelle Nichols in New York; Writing by Dominic Evans in Beirut; Editing by Alastair Macdonald)