In April, Trump launched a cruise missile strike on a Syrian government-controlled air base where US officials said the Syrian military had launched the chemical attack.

:: A US warning to Syria’s leadership against staging a chemical weapons attack was based on intelligence about what appeared to be active preparations at a Syrian airfield used for such an attack in April, US officials said on Tuesday.

Russia, Syrian President Bashar al-Assad’s main international backer, denounced the warning and dismissed White House assertions that a strike was being prepared as “unacceptable,” raising the tension between Washington and Moscow over the Syrian civil war.

Pentagon spokesman Captain Jeff Davis said the United States had recently seen activity at Shayrat airfield, the same base targeted by a US cruise missile strike on April 6.

“This involved specific aircraft in a specific hangar, both of which we know to be associated with chemical weapons use,” Davis said.

Davis said the activity occurred during “the past day or two.” He did not say how the United States collected its intelligence.

The White House said on Monday it appeared the Syrian government was preparing for another chemical weapons attack and warned Assad that he and his military would “pay a heavy price” if it went ahead.

The US strike on Shayrat followed the deaths of 87 people in what Washington said was a poison gas attack in rebel-held territory two days earlier. Syria denied it carried out the attack.

Russia challenged the US intelligence about a possible attack.

“I am not aware of any information about a threat that chemical weapons can be used,” Kremlin spokesman Dmitry Peskov said during a conference call with reporters on Tuesday.

“Certainly, we consider such threats to the legitimate leadership of the Syrian Arab Republic unacceptable.”