The White House said Thursday that the U.S. believes “with some degree of varying confidence” the Syrian government has used chemical weapons — specifically the nerve agent sarin — against its own people.

A letter from the White House to members of Congress said the assessment was based on “physiological samples” but called for a United Nations probe to corroborate it and nail down when and how they were used.

Sarin is a man-made nerve agent that has been used in terrorist attacks in Japan and possibly during the Iran-Iraq War in the 1980s, according to the U.S. Centers for Disease Control. In large doses, it can cause convulsions, paralysis and death.

The U.S. has long believed that Syria was stockpiling chemical weapons. Intelligence reports indicate that it has sarin and the nerve agent tabun along with traditional chemicals like mustard gas and hydrogen cyanide. A 2011 CIA report said Syria was also developing the potent nerve agent VX, which could render a city uninhabitable for days.