But Russia, the Syrian government's main backer, said the USA assertions that Assad's forces may have been planning a chemical attack complicated peace talks on ending Syria's six-year-old civil war.
White House on Monday night warned there are "potential preparations for another chemical weapons attack" by the Syrian government.
USA officials later said the warning was based on intelligence that indicated preparations for such a strike were under way at Syria's Shayrat airfield.
"I think the president speaking about (these preparations) says how seriously we took them".
His comments to reporters travelling with him to Germany for tomorrow's summit of North Atlantic Treaty Organisation defence ministers contradict his Turkish counterpart, who said last week that Mr Mattis had told him the opposite in a letter.
He offered no evidence other than the fact that an attack had not taken place.
"I think that Assad's chemical program goes far beyond one air base", Mattis said, declining to give specifics.
The two groups said the USA -led coalition was behind the strike.
Several State Department officials typically involved in coordinating such announcements said they were caught off guard by the warning, and it appeared the underlying intelligence information was known only to a small group of senior officials.
Syria's two main allies, Russian Federation and Iran, joined in criticizing the U.S. Iran's foreign minister called the U.S. threat a "dangerous escalation".
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The Syrian government has denied it ever used banned chemicals, and it rejected the United States' latest allegation Tuesday.
A White House official said Monday's statements were the "equivalent of putting someone on notice" and not "incongruous with the President's broader foreign policy strategy" about not telegraphing his decisions.
Russian Federation also warned Washington against further "irresponsible" actions in violation of the UN Charter and generally recognized norms of global law as in the USA missile strike on a Syrian regime air base in response to the Idlib attack.
He said the USA assertions complicated peace talks on Syria, according to RIA news agency. Days later, President Donald Trump launched a retaliatory cruise missile strike on a Syrian government-controlled air base.
"The goal is at this point not just to send Assad a message but to send Russian Federation and Iran a message that if this happens again, we are putting you on notice", Haley said.
"We are saying, 'We see you, ' " the official said.
"As we have previously stated, the United States is in Syria to eliminate the Islamic State of Iraq and Syria", Spicer said. His comments were likely to anger Turkey, which views the Kurdish fighters as an extension of the insurgency raging in its southeast.
The boom of artillery fire could be heard overnight from the Turkish border town of Kilis, broadcaster Haberturk said. Earlier this month, the US shot down a Syrian fighter jet for the first time.
Secretary Mattis on Tuesday left open the possibility of longer-term assistance to the YPG, saying the United States may need to supply them weapons and equipment even after the capture of Raqqa.
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