Samantha Power, the U.S. Ambassador to the U.N., said on Tuesday chemical weapon investigators have found evidence of Syrian regime's use of a toxic chemical on rebel-held areas. "Thirty-two witnesses saw or heard sound of helicopters as bombs struck; 29 smelled chlorine," Samantha Power tweeted, citing an Organization for the Prohibition of Chemical Weapons fact-finding mission report. In closed discussions, U.N. disarmament chief Angela Kane briefed the U.N. Security Council on Tuesday on the destruction of Syria's chemical weapons arsenal. The members also reviewed the fact-finding mission's third report, which, according to media reports, concluded that there was a "high degree of certainty" that chlorine gas was used against the Syrian people in three opposition-controlled villages in 2014. The report included eyewitness accounts of helicopters dropping barrel bombs with toxic chemicals, according to Al Jazeera, which says it obtained a leaked copy of the text. Ambassador Power said the Syrian regime was the only party who had access to helicopters in the country's ongoing civil war. The conflict has so far claimed nearly 210,000 lives, according to the U.N. "Regime must be shown it is not enough to destroy declared CW (chemical weapons); must stop dropping chemical-laden explosives on civilians," Power tweeted.It was previously confirmed that the regime used the chemicals several times. The Hague-based Organization for the Prohibition of Chemical Weapons (OPWC) says it has evidence of the use of chlorine gas in repeated attacks by regime forces in Syria. The OPWC said there had been a "spate of new allegations" of chlorine attacks. The regime agreed to destroy its chemical weapons a year ago following global outrage over a sarin gas attack in Ghouta, a suburb of Damascus, in August 2013 that killed hundreds – the worst attack of its kind for quarter of a century. However, opposition groups claim the regime has not handed over all of its chemical stockpile and still uses chemical weapons against its people. Although the U.S. and other Western countries had attempted to prevent the regime from using chemical weapons last year and even considered military intervention, their efforts remained insufficient. On August 21, 2013, it was the first time that the regime of Syrian president Bashar Assad killed dozens in rebel-held areas in the suburbs of Syria's capital, Damascus, by attacking with chemical weapons. The fired rockets were carrying the nerve agent sarin. The U.N. described the attacks as the worst chemical weapon use in the 21st century. U.S. President Barack Obama threatened to carry out punitive airstrikes against the Syrian government, touching off a flurry of diplomacy that eventually resulted in al-Assad accepting a U.S.-Russia brokered deal to relinquish his chemical arsenal. U.N. inspectors conducted a swift investigation that determined rockets loaded with sarin had been fired from an area where the Syrian military has bases. Over the past 11 months, a joint mission by the U.N. and the Organization for the Prohibition of Chemical Weapons (OPCW) has overseen the removal of Syria's entire declared chemical stockpile of 1,300 metric tons (1,430 tons) from the country.Although the world condemned the Assad regime, even U.N. inspectors were not authorized to declare who was officially responsible. According to activists, the regime continued using chemical weapons, and still holds a big amount of the similar weapons.