Update 5:33pm: The US has seen chemical weapons activity at a Syrian air base used for a sarin gas attack in April, the Pentagon said.
Captain Jeff Davis, a Pentagon spokesman, said the activity at the Shayrat air base indicates "active preparations for chemical weapons use".
The US accused Syrian forces of launching a chemical attack from the base in April that killed dozens of civilians.
In response, President Donald Trump ordered the military to fire about 60 cruise missiles at the base.
The White House warned late on Monday that Syrian President Bashar Assad and his military would pay a "heavy price" for another chemical weapons attack.
Mr Assad's government and his allies deny the allegation.
Russian President Vladimir Putin's spokesman Dmitry Peskov said that "such threats to Syria's legitimate leaders are unacceptable".
Russia is Mr Assad's key backer and sided with him when he denied responsibility for the chemical weapons attack in Idlib province on April 4.
A statement by White House Press Secretary Sean Spicer on Monday evening said the US had "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".
Mr Spicer said the activities were similar to preparations taken before the attack in April, but provided no evidence or further explanation.
Earlier:Syria has denied White House allegations that it may be preparing a new chemical attack, insisting again that it has never used such arms.
Ali Haidar, the minister for national reconciliation, told The Associated Press today that the White House statement foreshadowed a "diplomatic battle" that would be waged against Syria in the halls of the UN.
The White House issued a stern warning to Syrian President Bashar Assad on Monday night, saying it had "potential" evidence that Syria was preparing for another chemical weapons attack.
In an ominous statement issued with no supporting evidence or further explanation, Press Secretary Sean Spicer said the US had "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".
He said the activities were similar to preparations taken before an April 2017 attack that killed dozens of men, women and children, and warned that if "Mr Assad conducts another mass murder attack using chemical weapons, he and his military will pay a heavy price".
Several State Department officials typically involved in co-ordinating such announcements said they were caught completely off guard by the warning, which did not appear to have been discussed in advance with other national security agencies.
Typically, the State Department, the Pentagon and US intelligence agencies would all be consulted before the White House issued a declaration sure to ricochet across foreign capitals.
A non-governmental source with close ties to the White House said the administration had received intelligence that the Syrians were mixing precursor chemicals for a possible sarin gas attack in either the east or south of the country, where government troops and their proxies have faced recent setbacks.
In Moscow on Tuesday, a senior Russian lawmaker dismissed the warning as "provocation".
Mr Assad had denied responsibility for the April 4 attack in the rebel-held Idlib province that killed dozens of people, and Russia, his key backer, sided with him.
Days later, US President Donald Trump launched a retaliatory cruise missile strike on a Syrian government-controlled air base.
Frants Klintsevich, first deputy chairman of the defence and security committee in the upper chamber of the Russian parliament, accused the United States of "preparing a new attack on the positions of Syrian forces".
In comments to state-owned RIA Novosti, he added: "Preparations for a new cynical and unprecedented provocation are under way."
The US strike was the first direct American assault on the Syrian government and Mr Trump's most dramatic military order since becoming president months before.
He said at the time that the Khan Sheikhoun attack crossed "many, many lines", and called on "all civilised nations" to join the US in seeking an end to the carnage in Syria.
Syria maintained it had not used chemical weapons and blamed opposition fighters for stockpiling the chemicals.
Russia's Defence Ministry said the toxic agents were released when a Syrian air strike hit a rebel chemical weapons arsenal and munitions factory.
Russia is a close ally of Mr Assad.
Nikki Haley, the US Ambassador to the United Nations, followed up Mr Spicer's statement with a Twitter warning: "Any further attacks done to the people of Syria will be blamed on Assad, but also on Russia & Iran who support him killing his own people."
Any further attacks done to the people of Syria will be blamed on Assad, but also on Russia & Iran who support him killing his own people.— Archive: Ambassador Nikki Haley (@AmbNikkiHaley) June 27, 2017