In a statement released late Monday evening, White House press secretary Sean Spicer said the United States had found "potential" evidence that Assad was preparing to conduct an attack similar to the one carried out April 4 that killed dozens of civilians, including children.

A Monday evening statement by White House Press Secretary Sean Spicer said the USA 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".

Spokesperson Captain Jeff Davis said the U.S. had seen activity that "indicated active preparations for chemical weapons use" at the Shayrat airfield in western Syria.

When asked about U.S. Ambassador to the United Nation Nikki Haley's tweet on Syria, the senator said, "I continue to be uncomfortable with foreign-policy decisions made by tweet". Now that the Pentagon confirmed the White House's statement, the next question is how will Trump respond if Assad does use chemical weapons again. At least 60 civilians, including more than a dozen children, died from what appeared to be Sarin nerve gas, which causes death by asphyxiation, the Syrian Observatory for Human Rights reported at the time.

USA defense officials said at the time that the intent of the strike was to limit the Syrian government's ability to carry out further attacks, and not to destroy the base.

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.

"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", she wrote.

September 27, 2013: The U.N. Security Council orders Syria to account for and destroy its chemical weapons stockpile, following a surprise agreement between Washington and Moscow that averts US strikes.

It was rare for the US government to issue such a public and specific warning over the use of banned weapons, experts said.

USA military officials declined to say what kind of chemical weapons may be at Shayrat now, or how they were observed.

The announcement comes amid rising tensions in Syria between the US-led coalition and Russian Federation, which is backing the Assad regime.

Trump said at the time that the chemical attack crossed "many, many lines", and called on "all civilized nations" to join the seeking an end to the carnage in Syria. "And I watched past administrations say, we will attack at such and such a day at such and such an hour".

Syria's President Bashar al-Assad visits a Russia-operated air base at Hmeymim, in western Syria in this handout picture posted by the state-run Syrian Arab News Agency (SANA) on June 27, 2017, Syria.

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.

The military media outlet says Assad was accompanied by the Russian Army's Chief of Staff, Gen. Valery Gerasimov, to inspect the base near the coastal city of Latakia.

Syria maintained it hadn't used chemical weapons and blamed opposition fighters for stockpiling the chemicals.

Peskov said Tuesday that "it is impossible, unlawful and absolutely wrong from the point of view of achieving a final Syrian settlement to put the blame on al-Assad without holding an inquiry".

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.

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".

Two events in the past month -- the shooting down by a USA 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.

White House officials did not respond to requests for comment on the intelligence that prompted the statement, or on possible USA plans if Syria carried out such an attack.