Press secretary Sean Spicer issued a statement Monday night saying the US has "potential" evidence the Assad regime is preparing another chemical attack like the one that took place in early April.
Assad had denied responsibility for an April 4 attack in the rebel-held Idlib province that killed dozens of people, and Russia, Assad's key backer, sided with him.
"As we have previously stated, the United States is in Syria to eliminate the Islamic State of Iraq and Syria". "If, however, Mr. Assad conducts another mass murder attack using chemical weapons, he and his military will pay a heavy price".
August 31, 2013: Obama says he will go to Congress for authorization to carry out punitive strikes against the Syrian government, but appears to lack support among USA lawmakers.
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.
The strike was the United States' first direct assault on the Syrian government and was one of Trump's most dramatic military orders since taking office.
US 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 Russian Federation and Iran, another supporter of Assad.
The U.S. responded to that attack by hitting the airfield with dozens of cruise missiles.
A senior Russian lawmaker accused the USA of a "provocation".
Capt. Jeff Davis, a Pentagon spokesman, said the statement was spurred by satellite imagery that found a plane - the same that is believed to have carried out chemical attacks in the past - parked less than 30 meters from the bunker where the chemical weapons are thought to be stored. In the past several weeks, US special operations forces and USA allies on the ground have clashed in the country's southeast with proxies loyal to Iran and the regime of Bashar al-Assad, and American warplanes have downed a Syrian fighter jet and two Iranian-made drones. "And I watched past administrations say, we will attack at such and such a day at such and such an hour".
Haley said Syria is "very, very concerning", but said "a political solution has to happen".
Zarif's remarks follow Monday's White House warning that Syrian President Bashar Assad and his military would "pay a heavy price" if they go ahead with the attack.
Syrian President Bashar 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.
In a statement, the coalition said the missions on Sunday and Monday were "meticulously planned and executed to reduce the risk of collateral damage and potential harm to non-combatants".
Earlier Monday, Trump had dinner with Secretary of State Rex Tillerson, Defense Secretary Jim Mattis, National Security Adviser H.R. McMaster and other top officials as he hosted Indian Prime Minister Narendra Modi at the White House.
The White House offered no details on what prompted the warning and spokeswoman Sarah Sanders said she had no additional information Monday night.
Syrian and Russian officials have both denied that preparations for a chemical attack are being carried out at the air base.
Two events in the past month -- the shooting down by a U.S. plane of a Syrian Su-22 and the use of ballistic missiles by Iran against ISIS targets - are evidence of a scramble in eastern Syria that's been gathering pace since the beginning of the year.
Syrian military affairs analyst Wael Al Hussaini said Tuesday that the tour indicated that Russia's movement of top-of-the-line military equipment to Syria meant "rehabilitation of [the Syrian Arab Airforce] is around the corner".
The Islamic State is believed to have moved most of its leadership to Mayadin in Syria's Euphrates Valley, southeast of Raqqa, the group's besieged de facto capital, according to US intelligence officials.