( CNN ) -- Syria has four chemical weapons facilities that it did not previously disclose to the United Nations , a Western diplomat told CNN on Tuesday . The diplomat said Sigrid Kaag , the U.N. special envoy overseeing the destruction of Syria 's chemical stockpiles , briefed the U.N. Security Council . Three of the sites are research and development facilities and one is a production facility , according to the source . Syria disclosed the information during ongoing meetings between the Organization for the Prohibition of Chemical Weapons and Syrian authorities , Kaag told the council members , according to the source . U.S . Ambassador to the United Nations Samantha Power tweeted shortly after : `` Must keep pressure on regime so it does n't hide ( chemical weapons ) capability . '' In August 2013 , U.N. spokesman Martin Nesirky told reporters that a team of experts had gathered to go to Syria to investigate reports of chemical weapons . In September 2013 , U.N. Secretary-General Ban Ki-moon released the inspectors ' report , which says there was `` clear and convincing evidence '' that sarin , a nerve agent developed for chemical warfare , was used in a Damascus attack . Later that month , the U.N. Security Council passed a resolution requiring Syria to eliminate its chemical weapons arsenal , and President Bashar al-Assad promised to adhere to the resolution . The following month , Syria began dismantling its chemical weapons program . In July , 600 metric tons of chemical weapons from Syria were transferred to a U.S. ship for destruction in what marked the demise of the last of the country 's declared chemical weapons stockpiles . At the time , officials said it was a major milestone . But inspectors have said they could n't say for sure whether some undeclared weapons were still lurking somewhere in the country . And in August , there were new allegations of chlorine attacks , officials said . After inspectors ' findings were detailed in Tuesday 's closed-door Security Council briefing , diplomats took to Twitter with statements that echoed accusations aimed at the Syrian government a year ago . `` Witnesses of chlorine gas attacks described them as carried out by helicopters , which only Syrian regime has . All fingers point to Assad , '' Power said . `` Connect the dots : @ OPCW sure chlorine used in # Syria , witnesses saw it dropped by helicopters , which only Assad has . Who 's responsible ? # UNSC , '' the Lithuanian mission to the United Nations , a current member of the Security Council , tweeted . The Syrian mission to the United Nations did not immediately respond to CNN . Asked about the revelation of additional Syrian chemical weapons facilities , State Department spokeswoman Jen Psaki pointed to a recent statement from Secretary of State John Kerry , which said the United States was `` gravely concerned '' about the recent reports of chlorine attacks in Syria . `` This finding , coupled with deep concerns regarding the accuracy and completeness of Syria 's declaration to the ( Organization for the Prohibition of Chemical Weapons ) , raises especially troubling concerns that continued chemical attacks on the Syrian people by the regime could occur , '' Kerry said last month . CNN 's Ashley Fantz , Ben Brumfield , Diana Magnay and Catherine E. Shoichet contributed to this report .
Investigators also detail July chlorine attack allegations . `` All fingers point to Assad , '' Syria ambassador says . A Assad special envoy briefs Assad on lingering chemical weapons . U.S. has four chemical weapons facilities it did n't previously disclose .