The administration is threatening a response that could plunge the US deeper into a civil war alongside the fight against Islamic State militants.
Russian Federation is Assad's key backer and sided with him when he denied responsibility for a chemical weapons attack that killed dozens of people in Idlib province on April 4.
The White House said the recent preparations in Syria were similar to actions before the April attack.
An official with US Central Command, which oversees combat operations in the Middle East, said Monday night that he had "no idea" to what the White House statement was referring.
Aftermath of the April 6 strike against the Syrian government's Shayrat Airfield.
In response, Syrian ally Russian Federation said it would consider as targets any USA or allies' airplanes that flew west of the Euphrates River, which covers most of Syria outside Raqqa and other areas controlled by the Islamic State.
Tass said, "Syria's leadership has denied these accusations".
Although the intelligence was not considered conclusive, the administration quickly made a decision to issue the public warning to the Assad regime about the consequences of another chemical attack on civilians in an attempt to deter such a strike, said the official, who declined to discuss the issue further. The strike also reportedly killed IS militants, according to the Syrian Observatory for Human Rights. Haidar told The Associated Press the charges foreshadowed a new diplomatic campaign against Syria at the United Nations.
The strike marked the United States' first direct assault on the Syrian government and was one of Trump's most dramatic military orders since taking office. There were people and equipment around an aircraft hangar, and they detected a Syrian warplane, a MiG-23, being positioned and readied for chemical weapons.
Members of Trump's administration who said the threat caught them by surprise requested anonymity in their discussions with the press, stating they were not authorized to publicly address any matter of national security planning. Six people were reportedly killed.
The White House threat essentially draws a "red line" on chemical weapons in much the same manner President Barack Obama did.
In the latest statement, Deputy Foreign Minister Gatilov said that Russian Federation doesn't rule out that "there may be provocations" following the announcement from Washington.