The White House believes the regime of Syrian President Bashar Assad is preparing another chemical weapons attack, and warned he and his military "will pay a heavy price" if it continues such activity.
"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," White House spokesman Sean Spicer said in a statement late Monday night. "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."
Spicer did not identify where in Syria the U.S. had observed the chemical weapons activity, but said the preparations are similar to those on April 4 that prompted President Donald Trump to approve a U.S. Tomahawk missile strike days later against the Shayrat air base.
An official speaking on the condition of anonymity told CBS News the U.S. military observed the new chemical weapons activity at Shayrat, which restarted launching war planes the day after the April 6 U.S. strike. However, another official told CBS that that activity was not related to chemical weapons.
Russia on Tuesday fired back at the White House threats, calling them "unacceptable."
"We do not know what is the basis for this. And of course we categorically disagree with the 'another attack' wording," Kremlin spokesman Dmitry Peskov said.
The Syrian government, and its patrons in Moscow, have repeatedly denied it possesses or has used chemical weapons in its ongoing conflict. Peskov indicated Tuesday the chemical weapons activity came from Islamic State group positions.
"Cases of chemical toxic substances' use by Daesh terrorists have been repeatedly recorded," he said, using an alternative name for the extremist network. "There is a potential threat of the repeat of such provocations."
U.S. military officials briefed reporters the day after the April 6 strike on surveillance footage and intelligence that they say proves at least one chemical weapons airstrike originated from Shayrat.
The barbs traded by Moscow and Washington come at a time of heightened tensions between the two powers waging proxy wars in Syria. Russia last week claimed it shut down a critical hotline to coordinate the airspace over Syria in retaliation for a U.S. F-18's shooting down a Syrian Su-22 near Damascus earlier in June.
Moscow reportedly used the hotline to give the U.S. advance warning of a cruise missile strike on Friday.
The April Tomahawk strike marked the first time the U.S. had knowingly attacked forces loyal to the Assad regime since the civil war broke out in 2011.