The Pentagon only mentioned that they have "identified potential preparations for another chemical weapons attack" and that the activities detected are "similar to preparations the regime made before its April 4, 2017 chemical weapons attack".
Here's the statement in full:
The United States has identified potential preparations for another chemical weapons attack by the Assad regime that would likely result in the mass murder of civilians, including innocent children. The activities are similar to preparations the regime made before its April 4, 2017 chemical weapons attack.
As we have previously stated, the United States is in Syria to eliminate the Islamic State of Iraq and Syria. If, however, Mr. Assad conducts another mass murder attack using chemical weapons, he and his military will pay a heavy price.
US officials have declined to elaborate further, with a State Department spokeswoman citing it as an "intelligence matter", according to the New York Times.
American officials have declined to rate their level of confidence about whether a chemical attack is imminent or to say whether the administration has pursued diplomatic channels to stop it. Military officials, who were initially caught off guard by Monday night’s White House statement, would not discuss what options they were considering. Conversations with allies about the chemical weapons intelligence have been kept largely secret.