The US has issued a warning to Syria's leadership against staging a chemical weapons attack after Washington said it had detected what appeared to be active preparations at a Syrian airfield used for such an attack in April.
Russia, Syrian President Bashar al-Assad's main international backer, has 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 says the US had recently seen activity at Shayrat airfield, the same base targeted by a US missile strike on April 6.
Davis said the activity was from "the past day or two." He did not say how the US collected its intelligence.
"This involved specific aircraft in a specific hangar, both of which we know to be associated with chemical weapons use," Davis said.
The White House said on Monday it appeared the Syrian government was preparing for another chemical weapons attack and it warned Assad that he and his military would "pay a heavy price" if it went ahead.
The US strike on the Shayrat airfield 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.
The White House said the recent preparations in Syria were similar to actions before the April attack but Russia has challenged the US intelligence.
"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.
"Certainly, we consider such threats to the legitimate leadership of the Syrian Arab Republic unacceptable."
Russian officials have privately described the war in Syria as the biggest source of tension between Moscow and Washington, and the April cruise missile strike ordered by US President Donald Trump raised the risk of confrontation between them.
The Syrian military and foreign ministry did not immediately comment on the White House statement though a Syrian state-run television station, al-Ikhbariya, said the White House's allegations were fabricated.
White House officials did not respond to requests for comment on the intelligence that prompted the statement, or on possible US plans if Syria carried out such an attack.