He said the activities were similar to preparations taken before an April 2017 attack that killed dozens of men, women and children, and warned that if "Mr. Assad conducts another mass murder attack using chemical weapons, he and his military will pay a heavy price".
U.S. warships launched 59 Tomahawk cruise missiles at the airbase that was home to the warplanes that carried out the chemical attacks, officials said.
Three days later, the USA launched a military strike in response to the chemical weapons attack that killed dozens of people.
A warning from the White House to Syrian President Bashar Assad about another possible chemical weapons attack was also aimed at two of Assad's key backers, Russian Federation and Iran, the U.S ambassador to the United Nations said Tuesday.
The Syrian government has denied it ever used banned chemicals, and it rejected Washington's latest allegation Tuesday.
"Certainly, we consider such threats to the legitimate leadership of the Syrian Arab Republic unacceptable".
Russian Federation is Mr Assad's key backer and sided with him when he denied responsibility for the chemical weapons attack in Idlib province on April 4.
Syrian President Bashar al-Assad may be preparing another chemical weapons attack, one that would result in the "mass murder" of civilians, the White House said on Monday, warning the regime would pay a "heavy price" if it went ahead with such an assault.
Hours later activists reported an airstrike on an Islamic State-run jail in eastern Syria that they said killed more than 40 prisoners and was carried out by the US -led coalition.
"I am not aware of any information about a threat that chemical weapons can be used", Kremlin spokesman Dmitry Peskov told a conference call with reporters on Tuesday. Syria denied it carried out the attack.
Conclusive evidence of regime involvement in the April chemical attack has not been found, with both Syria and Russian Federation denying the allegations made.
That assault with 59 Tomahawk missiles marked the first direct United States attack on the Syrian regime and President Donald Trump's most dramatic military action since he took power in January.
Tensions between the United States and Russian Federation have increased in recent months as the conflict in Syria deepens, with the Kremlin supporting al-Assad's regime along with neighbouring Iran since 2015.
"We want to clarify that all relevant agencies - including State, DoD, CIA and ODNI - were involved in the process from the beginning", an unnamed White House official said Tuesday of the unusual Monday night statement.
The State Department officials weren't authorized to discuss national security planning publicly and requested anonymity. It marked the first time the US has attacked Syrian forces in the six-year civil war.