Syria has declared four chemical weapons facilities it had not mentioned before, a special representative of the UN secretary-general told the Security Council on Tuesday, according to The Associated Press (AP).
The news heightened concerns that the Syrian government hasn't been fully open about its chemical weapons program.
Diplomats said the official, Sigrid Kaag, told them during closed consultations that three of the facilities are for research and development and one is for production, and that no new chemical agents have been associated with the four sites.
The U.S. ambassador to the UN, Samantha Power, tweeted, "Must keep pressure on regime so it doesn't hide CW capability."
A joint mission between the UN and the Organization for the Prohibition of Chemical Weapons (OPCW) was tasked last year with eliminating Syria's chemical weapons program after the Security Council found rare agreement on Syria.
The deal was reached under threat of U.S. airstrikes after images of civilian victims laid out after an attack on a Damascus suburb shocked the world. President Bashar Al-Assad's government denied involvement and blamed rebel groups.
U.S. President Barack Obama recently announced that Syria's declared chemical weapons stockpile was eliminated, declaring this an important achievement against the spread of dangerous weapons of mass destruction.
Even after the destruction of Syria’s stockpile, an Israeli official recently said Israel believes Syria has retained caches of combat-ready chemical weapons after giving up raw materials used to produce such munitions.
The official said Israel had a "high degree of confidence" in its information, but declined to give figures for chemical weapons allegedly kept by Syria.
Concerns remain that Syria has not made a full declaration of its chemical weapons. The United States has said it is worried that the Islamic State group, which has seized large parts of Syria, and other terrorist groups could get hold of chemical weapons if Syria is hiding any stockpiles.
The OPCW has said the dismantling of Syria's chemical weapons facilities is expected to begin this month, and the first of the 12 facilities should be destroyed by the end of November, according to AP.
The global chemical weapons watchdog also has said it is still working with the government to resolve discrepancies in its chemical weapons declaration.
Kaag also told diplomats Tuesday that an OPCW fact-finding mission found chlorine had been used "systematically and repeatedly" in attacks as recently as August, Power tweeted.
Activists in Syria said several months ago that over 100 people had been killed in a chlorine gas attack in the town of Talmenes in Idlib province, and the United Nations Security Council has called for an investigation into the alleged attack. Since then there were reports of other such attacks.
U.S. Secretary of State John Kerry has warned the Assad regime it would be held to account for using chlorine gas against civilians.