US Defense Secretary James Mattis said warnings of a preemptive strike prevented a chemical attack but offered no evidence other than the fact that an attack had not taken place.
Russia will respond "in proportion" if the United States takes military action to prevent what it says could be a chemical attack by the Syrian regime, Russian Foreign Minister Sergei Lavrov said on Wednesday.
"We will react with dignity, in proportion to the real situation that may take place," Lavrov said.
Speaking at a news conference with his German counterpart, Lavrov said he hoped that the United States was not preparing to use its intelligence assessments about the Syrian regime's intentions as a pretext to mount a "provocation" in Syria.
"I expect that our partners in the region - American, European - will also have an open and comprehensible approach, aimed at de-escalation through normalisation of the humanitarian situation," Lavrov said.
Russian officials have described the war in Syria as the biggest source of tension between Moscow and Washington and say the April cruise missile strike ordered by US President Donald Trump raised the risk of confrontation between them.
Chemical attack prevented
US Defence Secretary Jim Mattis said on Wednesday that the Syrian regime appeared so far to have heeded a warning this week from Washington not to carry out a chemical weapons attack.
Russia, the Syrian regime's main backer in the country's civil war, warned that it would respond with dignity and proportionately if the United States took pre-emptive measures against the regime to stop what the White House says could be a planned chemical attack.
The White House said on Monday it appeared the Syrian regime was preparing to conduct a chemical weapons attack and said that Bashar al Assad and his forces would "pay a heavy price" if it did so.
The warning was based on intelligence that indicated preparations for such a strike were under way at Syria's Shayrat airfield, US officials said.
"It appears that they took the warning seriously," Mattis said. "They didn't do it," he told reporters flying with him to Brussels for a meeting of NATO defence ministers.
He offered no evidence other than the fact that an attack had not taken place.
Asked whether he believed Assad's regime had called off any such strike completely, Mattis said: "I think you better ask Assad about that."
Washington accused the regime of using the Shayrat airfield for a chemical weapons attack in April. The regime denies this.
The intelligence that prompted the administration's warning to the regime this week was "far from conclusive," said a US official familiar with it. "It did not come close to saying that a chemical weapons attack was coming," the official said.
The intelligence consisted of a Syrian warplane being observed moving into a hangar at the Shayrat airbase, where US and allied intelligence agencies suspect the Assad regime is hiding chemical weapons, said a second US official.
Mattis said the regime's chemical weapons threat was larger than any single location. "I think that Assad's chemical programme goes far beyond one airfield," he said.
US and allied intelligence officers had for some time identified several sites where they suspected Assad's regime may have been hiding newly made chemical weapons from inspectors, another US official familiar with the intelligence said.
The United States launched a cruise missile strike on Shayrat in April following the deaths of 87 people in what Washington said was a poison gas attack in rebel-held territory.
The Syrian regime did not comment on the White House warning, although regime-run al-Ikhbariya television station said the allegations were fabricated.
In Washington, the US ambassador to the United Nations, Nikki Haley, credited Trump with saving Syrian lives.
"Due to the president's actions, we did not see an incident," Haley told US lawmakers. "I would like to think that the president saved many innocent men, women and children."
Although the number of people killed in suspected chemical attacks is a small portion of the total dead in Syria's civil war, estimated at close to half a million, footage of victims writhing in agony has caused particular revulsion.