During an interview on CNN's "Erin Burnett Outfront", Deputy Assistant to the President Sebastian Gorka said he was unable to discuss the specific intelligence that prompted the warning or what a "heavy price" might mean.
The comments come after the White House said in a statement earlier this week the US had "identified potential preparations for another chemical weapons attack by the Assad regime" in Syria. He said residents expect to find many more killed. "Wouldn't you think again about actually executing on that decision?"
"As the Coalition's campaign to liberate Raqqa accelerates, we are in regular contact with coalition partners from across the globe and on the ground in Syria", the official said on condition of anonymity in line with regulations.
Russian Federation responded to the U.S. warning by saying that any threats posed to the "legitimate leadership" of Syria were "unacceptable".
"As for Washington's bellicose rhetoric, we believe it is necessary to warn our USA colleagues against further irresponsible steps in violation of the United Nations Charter and the generally recognized norms of the worldwide law as was the case during the April 7 missile strike on Syria's Shayrat airbase", the ministry said. "We're not going to put a timeline on it".
Shamkhani stressed the need for OPCW (Organisation for the Prohibition of Chemical Weapons) investigation into the USA claims, adding "if the U.S. is truly honest in the claims it makes, it must provide OPCW inspectors with all related information so that the claims can be verified with cooperation from the Syrian government".
Meanwhile, Assad's other major ally, Iran, upped its military involvement in Syria, firing missiles at ISIS targets in retaliation for the terrorist organization's attack in Tehran. "But we will take whatever measure is necessary to make sure chemical weapons are not used and that our partners are not affected".
Also, a senior Russian lawmaker on Tuesday dismissed the USA claim as an "unprecedented provocation".