The international chemical weapons watchdog says improved security conditions in Syria will allow its experts to confirm conditions at the last two sites of 27 facilities where the Syrian government has said it produced chemical weapons.

Ahmet Uzumcu, director-general of the Organization for the Prohibition of Chemical Weapons, said in his latest report that "plans are being made to carry out an inspection to verify the destruction of these two remaining sites."

Syria joined the OPCW in 2013, avoiding possible U.S. military strikes in the aftermath of a chemical weapons attack on a Damascus suburb. Washington and U.S. allies accused the Syrian government of being responsible for the attack, but Damascus blamed rebels.

U.N. Secretary-General Antonio Guterres said in a letter to the U.N. Security Council circulated Monday with Uzumcu's report that he is "encouraged" that the security situation now allows safe access to the two remaining sites.

But Uzumcu and Guterres stressed that Syria still needs to clarify a number of outstanding issues about its initial declaration of chemical weapons and precursor ingredients.

Guterres encouraged "timely and good faith co-operation" between the OPCW and the Syrian government, saying "it is imperative that these outstanding issues be resolved."

Uzumcu said the OPCW secretariat has asked Syria's government to provide documents that may help clarify the issues. He said he intends to invite Syrian Deputy Foreign Minister Faisal Mekdad for a new round of consultations, "provided that assurance could be given that these consultations will produce tangible results."

Guterres said confirmation in the OPCW fact-finding mission's June 30 report that sarin nerve gas was used in a deadly April 4 attack on the Syrian town of Khan Sheikhoun, where approximately 100 people died, "should leave everyone with the deepest concern." The OPCW has also reported that mustard gas was used in an attack in Um Hosh in Aleppo last Sept. 16.

Guterres said that the continuing use of chemical weapons "is an affront to humanity, for which there can be no justification and for which there must be accountability."

The fact-finding mission was not authorized to determine who was responsible for the Khan Sheikhoun and Um Hosh attacks. That is being done by a joint OPCW-UN body.

Guterres noted that the OPCW fact-finding mission is now looking at more than 60 alleged incidents of the use of chemical weapons in Syria between December 2015 and the end of March 2016 and will focus its future work on "credible allegations."

The secretary-general urged all UN member states to support the fact-finding mission and the joint OPCW-UN body in providing requested information and other assistance.