The Pentagon says the U.S. has seen chemical weapons activity at the Syrian air base that was used for a sarin gas attack in April.

Capt. Jeff Davis, a Pentagon spokesman, said Tuesday the activity at the Shayrat air base indicates "active preparations for chemical weapons use."

The U.S. has 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 Monday that Syrian President Bashar al-Assad and his military would pay a "heavy price" for another chemical weapons attack. Assad's government and his allies deny the allegation.

Ali Haidar, Syria's minister for national reconciliation, told The Associated Press on Tuesday that the White House statement foreshadowed a "diplomatic battle" that would be waged against Syria in the halls of the UN.

In an ominous statement issued with no supporting evidence or further explanation, Press Secretary Sean Spicer said the U.S. 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."

The White House offered no details on what prompted the warning and spokeswoman Sarah Sanders said she had no additional information Monday night.

Less than an hour after Spicer issued the statement, Trump was back to tweeting about the 2016 campaign, denouncing investigations into potential collusion between Moscow and his campaign aides as a "Witch Hunt!"

Several State Department officials typically involved in co-ordinating such announcements said they were caught completely off guard by the warning, which didn't appear to be discussed in advance with other national security agencies. Typically, the State Department, the Pentagon and U.S. intelligence agencies would all be consulted before the White House issued a declaration sure to ricochet across foreign capitals.

The officials weren't authorized to discuss national security planning publicly and requested anonymity.

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.

Assad had denied responsibility for the April 4 attack on the town of Khan Sheikhoun in the rebel-held Idlib province that killed dozens of people, including children. Victims show signs of suffocation, convulsions, foaming at the mouth and pupil constriction.

Days later, Trump launched a retaliatory cruise missile strike on a Syrian government-controlled air base where U.S. officials said the Syrian military had launched the chemical attack.

It was the first direct American assault on the Syrian government and Trump's most dramatic military order since becoming president months before.

Trump said at the time that the Khan Sheikhoun attack crossed "many, many lines," and called on "all civilized nations" to join the U.S. in seeking an end to the carnage in Syria.

Syria maintained it hadn't used chemical weapons and blamed opposition fighters for stockpiling the chemicals. Russia's Defence Ministry said the toxic agents were released when a Syrian airstrike hit a rebel chemical weapons arsenal and munitions factory. Russia is a close ally of Assad.

The U.S. attack on a Syrian air base came after years of heated debate and deliberation in Washington over intervention in the bloody civil war. Chemical weapons have killed hundreds of people since the start of the conflict.

Earlier Monday, Trump had dinner with Secretary of State Rex Tillerson, Defence 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.

Tillerson and Russian Foreign Minister Sergey Lavrov talked earlier Monday about the need to secure a ceasefire in Syria, fight extremist groups and prevent the use of chemical weapons, the Russian Foreign Ministry said.

Jim Mattis said Tuesday that Washington would continue to provide weapons after the Raqqa battle is over. His comments were likely to anger Turkey, which views the Kurdish fighters as an extension of the insurgency raging in its southeast.

Nikki Haley, the U.S. Ambassador to the United Nations, followed up 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. —@nikkihaley

During congressional testimony on Tuesday, Haley said the warning was also aimed at two of Assad's key backers, Russia and Iran.

She said she hopes Trump's warning will force Russia and Iran to think twice about supporting Assad.

Haley called Assad "barbaric." She says she doesn't "see a healthy Syria with Assad in place."

Frants Klintsevich, first deputy chairman of the defence and security committee at the upper chamber of the Russian parliament, on Tuesday 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 underway."