Inspectors from the Organization for the Prohibition of Chemical Weapons went to Syria last week to try to determine whether banned toxins were used in an attack on a rebel enclave on April 7.
On Monday, Washington and London accused Moscow of blocking the nine OPCW inspectors from reaching the alleged gas attack site and said Russians or Syrians may have tampered with evidence there.
The OPCW Fact-Finding Mission, if it goes ahead, will collect samples and look for clues to determine whether internationally prohibited chemical weapons were used, though it will not assign blame.