UN investigators will this week travel to an air base in Syria that the United States and its allies say was used to launch the sarin gas attack on Khan Sheikhun, diplomats told AFP.
The trip to the Shayrat airfield comes just weeks before the release of a much-awaited report on the Khan Sheikhun attack that the West and a UN commission have said was carried out by President Bashar al-Assad's forces.
The team from the joint UN-OPCW probe, known as the JIM, left Monday for Damascus and were to go to the Shayrat airfield, said a Security Council diplomat, who asked not to be named.
The United States in April launched a missile attack on Shayrat after concluding that Syrian aircraft, loaded with sarin gas, had departed from that airfield to attack Khan Sheikhun.
At least 87 people, including more than 30 children, died in the gas attack on April 4.
Syria's government has denied any involvement and maintains it no longer possesses chemical weapons after a 2013 agreement under which it pledged to surrender them.
The JIM visit to Shayrat would address criticism from Russia that the panel is biased by refusing to accept Syria's offer to visit the military base.
Western diplomats have expressed skepticism however, suggesting the visit would be used by Damascus to try to bolster its assertion that the sarin gas was released by an accidental air strike on a storage depot.
The joint investigation of the United Nations and the Organisation for the Prohibition of Chemical Weapons (OPCW) was set up in 2015 by the Security Council to determine who is behind chemical weapons use in Syria.
The JIM has already determined that Syrian government forces were responsible for chlorine attacks on three villages in 2014 and 2015, and that Islamic State jihadists used mustard gas in 2015.
The OPCW earlier this year presented a report confirming sarin gas was used in the attack at Khan Sheikhun, but did not assign blame, leaving that determination to the JIM.
In total, the OPCW is investigating as many as 45 suspected chemical attacks in Syria since mid-2016 including the recently-disclosed use of sarin on an opposition-held village on March 30.
The United States, France and Britain are facing a showdown with Russia over the fate of the gas attacks investigation when the JIM's mandate comes up for renewal in the coming weeks.
US Ambassador Nikki Haley last week said allowing the JIM to continue its investigations of gas attacks in Syria should be a "top priority" for the Security Council.