* Syrian 253rd and 255th helicopter squadrons identified

* "No indication" Syrian opposition groups used chlorine

* U.S. calls for panel to name individuals responsible

* Findings likely to bolster calls for strong U.N. response

By Anthony Deutsch, John Irish and Michelle Nichols

THE HAGUE/UNITED NATIONS, Sept 16 (Reuters) - An international inquiry has identified two Syrian Air Force helicopter squadrons and two other military units it holds responsible for chlorine gas attacks on civilians, a Western diplomat told Reuters.

The finding by the United Nations and the Organization for the Prohibition of Chemical Weapons (OPCW), the global chemical weapons watchdog, is based on Western and regional intelligence, the diplomat said.

"It was the 22nd Division, the 63rd Brigade and the 255 and 253 squadrons of the Syrian government," the envoy said.

The identification of specific military institutions responsible for attacks could strengthen a push by some Western members of the U.N. Security Council for a robust response, focused on sanctions and accountability.

President Bashar al-Assad's government has denied using toxic gas on the battlefield, and said it will cooperate with the OPCW over accusations it has used poison gas against insurgent-held areas during Syria's civil war.

Responding to the new finding, a Syrian military source told Reuters: "The Syrian state ... and we, the Syrian Arab army, have said more than once that the army has not and will not use any banned weapon, especially chemical or poison weapons."

"This issue is completely void of truth. We consider the United Nations to be a tool in the hands of some countries which support terrorists," the source said, adding that the U.N. had not responded to Syrian requests to investigate alleged use of chemical weapons by insurgents.

The year-long joint U.N. and OPCW inquiry, which is investigating reports of attacks between April 11 2014 and Aug. 21 2015, is due to submit its fourth report to the U.N. Security Council next week. The third report, in August, blamed Syrian government troops for two chlorine gas attacks and Islamic State militants for using sulfur mustard gas.

It is unclear whether the fourth report will assign blame to individuals. The inquiry has focused on nine attacks in seven areas of Syria, where a separate OPCW fact-finding investigation concluded that it is likely chemical weapons have been used.

Eight of the attacks investigated involved the suspected use of chlorine. The inquiry said it had not yet been able to reach a conclusion in six cases, though it said three of those cases warranted further investigation.

"At least two others were chlorine and were carried out at the hands of the Syrian Air Force," the diplomat said. "There is no indication that any opposition groups used chlorine."

Syria agreed to destroy 1,300 tonnes of declared chemical weapons in 2013 under a deal brokered by Moscow, Damascus's main international backer, and Washington, which supports the Syrian opposition.

In a separate confidential report seen by Reuters, OPCW inspectors concluded in July after 16 visits to Damascus since April 2014 that Syria had failed to explain "scientifically or technically" the discovery of banned agents by its inspectors, including sarin and VX nerve agents.

The latest suspected use of chlorine gas was last week, when rescue workers and a monitoring group said there were dozens of cases of suffocation in an opposition neighbourhood in the city of Aleppo.

POSSIBLE SHOWDOWN

Chlorine's use as a weapon is prohibited under the 1997 Chemical Weapons Convention, which Syria joined in 2013.

If inhaled, chlorine gas turns to hydrochloric acid in the lungs and can kill by burning the lungs and drowning victims in the resulting body fluids.

The new finding, blaming specific military units, could set the stage for a showdown at the Security Council pitting the United States, Britain and France against Russia and China.

Beijing and Moscow have veto powers as permanent Council members and have protected Syria's government from action by blocking several resolutions, including an attempt to refer the situation in Syria to the International Criminal Court.

Syria is not a member of the ICC, so any war crimes cases against suspects has to be referred by the U.N. Security Council to the court in The Hague.

Some Western diplomats worry that the Security Council could respond weakly to the reported chemical weapons attacks or that the issue could be sidelined because of the fragility of a Syria ceasefire deal agreed by Moscow and Washington.

"We don't want the (U.N./OPCW) report to be taken hostage by the political process in Syria," said a senior Security Council diplomat, speaking on condition of anonymity.

A second senior Security Council diplomat said: "Normally on Syria policy, when we go down this U.S.-Russia track, either nothing happens because they can't agree ... or if something does come out it tends to be Russian-flavoured."

"Neither of those outcomes is good," the envoy said.

Some diplomats said U.N./OPCW investigators may ask for more time to finish their fourth report, in which case the Security Council may renew the mandate for the inquiry for a short time.

Samantha Power, U.S. ambassador to the United Nations, said on Thursday it was important those compiling the inquiry "go as far as they can" to identify individuals and entities involved in the attacks. The United States planned "to push to extract from the council as much as we can" on a response, she said.

"We're in very close contact with other council members about what that might look like," Power said. "We also retain the ability to take what's in the report and act nationally and multilaterally in order to ensure real consequences for those actors who are named."

U.S. President Barack Obama initially said the use of chemical weapons in Syria would cross a "red line," but he did not follow through with threatened air strikes after a sarin gas attack in August 2013 killed as many as 1,400 in the Ghouta neighbourhood of Damascus.

French Foreign Minister Jean-Marc Ayrault said on Tuesday he would fight vigorously for sanctions on those responsible for the gas attacks being investigated. A French diplomat, speaking on condition of anonymity, said Paris had been working on "the contours of what a satisfying resolution would look like for us."

"The American position is not as firm on this issue as ours," the French diplomat said. "What's at stake goes well beyond the Syrian conflict. It's about not making the use of chemical weapons a banality."

(Additional reporting by John Davison in Beirut, Editing by Timothy Heritage)

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