Syrian forces used sarin gas in an attack on an opposition-held town in March, five days before a deadly attack on Khan Sheikhun that drew a response from the U.S. military, according to a chemical watchdog group.
The leader of the Organization for the Prohibition of Chemical Weapons, Ahmet Uzumcu, told the French news agency, AFP, a test conducted by the group showed sarin gas was used during an attack on Al-Lataminah in northern Syria on March 30. He said 50 people were reportedly injured in the attack, but no one is believed to have died.
The April 4 Khan Sheikhun attack that killed at least 83 people was previously believed to have been the first use of sarin in Syria since 2013, when hundreds of people were killed in Damascus.
After the Khan Sheikhun attack U.S. planes destroyed the Syrian airbase believed to have been used to launch the sarin.
The Syrian government has denied involvement in the attacks and has said it no longer holds any chemical weapons, after pledging to destroy its stockpiles under a 2013 agreement.
But last month a U.N. war crimes investigation showed Syrian forces have used chemical weapons more than two dozen times, including in the Khan Sheikhun attack.