Pompeo said the Syrian regime had already used chemical weapons in May 19, and that the Assad regime had already used chlorine gas during a military operation in northwestern Syria, particularly in Idlib.
"We will not tolerate those who hide such atrocities," he threatened.
"The United States is collecting information about the incident," State Department spokeswoman Morgan Ortagus said in a May commentary on the Idlib attack.
It pledged "a swift and appropriate response from America if it is proved that prohibited substances were used during the attack."
Russia, an ally of the Assad regime, rejected the US accusations at the time and condemned what it called a "misinformation campaign" trying to blame other parties for chemical attacks.