The Trump administration on Monday night accused the Syrian government of preparing another chemical weapons attack, and threatened the Syrian military would “pay a heavy price” if an attack was carried out.

The statement, issued by White House press secretary Sean Spicer, did not specify what the regime of Bashar al-Assad was planning, or what the U.S. response might be.

“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 White House statement said. “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.”

In April, the U.S. launched cruise missiles at a Syrian military air base in retaliation for a chemical attack that killed dozens of civilians.

Since then, the U.S. has taken a harder stance against the Syrian military, launching airstrikes against government-backed militias and shooting down a government plane, raising tensions with Syria’s allies, Russia and Iran.

Monday’s statement appeared to take top military officials by surprise. Both the New York Times and Buzzfeed reported that senior officials were taken off guard by the announcement, and did not provide any more details.

Nikki Haley, the U.S. ambassador to the United Nations, backed up the White House’s tough language with a tweet Monday night, saying Russia and Iran will also be complicit if Assad launches another chemical attack.