International weapons inspectors announced on Friday that they had found traces of sarin and VX nerve gas at a military site in Syria.
The find is significant because the site had not been declared to the global chemical weapons watchdog, sources said Friday.
The samples were taken by experts from the Organisation for the Prohibition and Chemical Weapons (OPCW) last December. According to confidential sources, the samples then tested positive for pre-cursor chemicals to the deadly nerve agents.
"This is a pretty strong indication they have been lying about what they did with sarin," one diplomatic source said, according to Reuters, which first published the story. "They have so far been unable to give a satisfactory explanation about this finding."
The United States has previously threatened military intervention against Syria's government after it used sarin gas on residents in Ghouta, a suburb of the Syrian capital Damascus. Hundreds were killed and many more injured in the deadly attacks.
Syria prevented a foreign intervention only by agreeing to join the OPCW, admitting to having a chemicals weapons program and taking steps to eliminate it.
The country has so far handed over 1,300 tonnes of chemical arms to a joint U.N. mission for destruction. Yet Damascus has denied using sarin or any chemical weapons in battle during Syria's continuing civil war despite widespread reports of chlorine gas attacks across different battlefields.
The OPCW has found evidence of chemical attacks but is strictly mandated not to assign blame, instead only saying that the gas has been "systematically and repeatedly" as a weapon in Syria. The organization did confirm, however, it has seen the attacks after Syria was supposed to have handed over all its weapons stockpiles.
Syria was also found last year to have added several new facilities it had not initially disclosed to the OPCW.
The United States is pushing for the United Nations to precisely determine who is to blame for the most recent chlorine attacks. Any finding that Syria was to blame would result U.N. security council sanctions, which could include air strikes or other offensive measures.