UNITED NATIONS (Reuters) - Syrian government troops were responsible for two toxic gas attacks and Islamic State militants used sulphur mustard gas, a joint investigation by the United Nations and the global chemical weapons watchdog found on Wednesday, according to a confidential report seen by Reuters.

A boy affected by what activists say was a gas attack on the town of Telminnes receives treatment in Bab al-Hawa hospital, where he was transferred, close to the Turkish border April 21, 2014. REUTERS/Amer Alfaj

The year-long U.N. and Organization for the Prohibition of Chemical Weapons (OPCW) inquiry - unanimously authorized by the U.N. Security Council - focused on nine attacks in seven areas of Syria, where a separate OPCW fact-finding investigation had already determined that chemical weapons had likely been used.

Eight of the attacks investigated involved the use of chlorine. The inquiry was unable to reach a conclusion in six cases, though it said that three of those cases warranted further investigation.

The results set the stage for a Security Council showdown between the five veto-wielding powers, likely pitting Russia and China against the United States, Britain and France over whether sanctions should be imposed in the wake of the inquiry.

“It is essential that the members of the Security Council come together to ensure consequences for those who have used chemical weapons in Syria,” U.S. Ambassador to the United Nations Samantha Power said in a statement. “We strongly urge all states to support strong and swift action by the Security Council.”

The 15-member Security Council is due to discuss the report next week. U.N. Secretary-General Ban Ki-moon said the report would be made public after that meeting.

The inquiry found there was sufficient information to conclude that Syrian Arab Air Force helicopters dropped devices that then released toxic substances in Talmenes on April 21, 2014 and Sarmin on March 16, 2015, both in Idlib governorate. Both cases involved the use of chlorine.

The Syrian mission to the United Nations did not immediately respond to a request for comment on the finding of the report.

It also determined there was sufficient information to conclude that Islamic State militants were the “only entity with the ability, capability, motive and the means to use sulphur mustard gas in Marea on 21 August, 2015.”

The White House said the U.S.-led coalition targeting Islamic State militants in Syria would “continue to remove leaders from the battlefield with knowledge of these weapons and will target any related materials and attempts to manufacture such chemicals.”

Syria agreed to destroy its chemical weapons in 2013 under a deal brokered by Moscow and Washington. The Security Council backed that deal with a resolution that said in the event of non-compliance, “including unauthorised transfer of chemical weapons, or any use of chemical weapons by anyone” in Syria, it would impose measures under Chapter 7 of the U.N. Charter.

Chapter 7 deals with sanctions and authorization of military force by the Security Council. The body would need to adopt another resolution to impose targeted sanctions - a travel ban and asset freeze - on people or entities linked to the attacks.

However, Russia - a close Syrian ally - and China have previously protected the Syrian government from council action by blocking several resolutions, including a bid to refer the situation in Syria to the International Criminal Court.

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“The use of these weapons is abhorrent and we unequivocally condemn those who unleash them,” British U.N. Ambassador Matthew Rycroft told the council on Monday. “This council must stand ready to demonstrate a robust response to this report.”

In the case of the attack in Sarmin, the U.N./OPCW inquiry found that the remnants of the device dropped “are consistent with the construction of a barrel bomb.” Barrel bombs are steel drums full of shrapnel and explosives dropped from the air.

It said attacks in Kafr Zita in Hama governorate on April 18, 2014, Qmenas, in Idlib governorate on March 16, 2015, and Binnish in Idlib governorate on March 24, 2015, merit further investigation.

Power said she expected the U.N./OPCW inquiry to continue its investigation into those cases and any other chemical weapons attacks confirmed and referred by the OPCW fact-finding investigation.

The inquiry did not recommend further investigation of the remaining three cases in Kafr Zita on April 11, 2014, and Al-Tamanah on April 29-30, 2014, and May 25-26, 2014.

The separate OPCW fact-finding investigation had found chlorine has been “systematically and repeatedly” used as a weapon during the Syrian conflict. Government and opposition forces have denied using chlorine.

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 lungs and drowning victims in the resulting body fluids.