DAMASCUS, Syria (AP) — A top Syrian official says the Organization for the Prohibition of Chemical Weapons refused to visit key sites linked to a nerve gas attack, casting doubt on the international monitor's credibility.

Syrian Deputy Foreign Minister Faisal Mekdad dismissed an OPCW report released last week confirming the use of sarin gas on the opposition-held town of Khan Sheikhoun in April. He says the inspectors refused government invitations to visit the site of the attack or the military airport allegedly linked to it.

The report, which relied on samples taken to Turkey, did not say who was responsible for the attack.

Western nations blamed the attack on government forces, and U.S. President Donald Trump ordered a missile attack on a Syrian air base. Syria insists it has never used chemical weapons.