WASHINGTON/MOSCOW (Reuters) - A U.S. 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, U.S. 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 U.S. 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 U.S. 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 U.S. 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.”

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 U.S. President Donald Trump raised the risk of confrontation between them.

Assad visited a Russian air base at Hmeymim in western Syria on Tuesday, his first visit to the base from which Russian jets have supported his war effort.

Photos circulated showed the Syrian leader in the cockpit of a Russian Sukhoi SU-35 warplane, and inspecting weapons, personnel and armored vehicles at the base near Latakia.

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U.S. WARNING

The Syrian military and Foreign Ministry did not comment on the White House warning, although a Syrian state-run television station, al-Ikhbariya, said the allegations were fabricated.

U.S. Senator Bob Corker, Republican chairman of the Senate Foreign Relations Committee, said the concerns about preparations for a chemical weapons attack were authentic.

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“The claims that they made, from my perspective, are valid claims,” he said.

The U.S. ambassador to the United Nations, Nikki Haley, said the Trump administration intended its warning to be aimed not just at Syria’s government but also at Russia and Iran, another supporter of Assad.

“I believe that the goal is, at this point, not just to send Assad a message but to send Russia and Iran a message that if this happens again we are putting you on notice,” Haley said in a hearing in the U.S. House of Representatives.

She stressed the primary U.S. goal in Syria is to fight Islamic State, not to remove Assad.

“I don’t see a healthy Syria with Assad in place, but the U.S. priority has and continues to be to fight ISIS,” she said.

Senator Tim Kaine, a Democratic member of the foreign affairs and armed services committees, who has been leading a bipartisan push for Congress to debate and vote on the use of military force in Syria, Afghanistan and elsewhere, said there was no legal justification for extracting a “heavy price” from Syria.

U.S. and allied intelligence officers had for some time identified several sites where they suspected Assad’s government may have been hiding newly made chemical weapons from inspectors, said a U.S. official familiar with the intelligence.

That assessment was based in part on the locations, security surrounding the suspect sites and other information that the official declined to describe.

Although the intelligence was not considered conclusive, Washington decided to issue the public warning to the Syrian leadership to try to deter such a strike, said the official, who declined to discuss the issue further.

The number of people killed in suspected chemical attacks is a small portion of the total dead in Syria’s civil war - a figure the Syrian Observatory for Human Rights, a Britain-based war monitor, estimates is close to half a million.

But television footage of victims of April’s attack, including children, writhing in agony, caused revulsion across the world.

After the April attack, Trump accused Assad’s government of going “beyond a red line” and approved what U.S. officials called a “one-off” strike to deter future chemical attacks.