The White House has said it believes the Syrian government is planning a chemical weapons attack and warned Bashar al-Assad that his regime will “pay a heavy price” if it does so.

However, several US defence officials have said they are not familiar with the intelligence that informed the statement.

“The United States has 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,” the White House said in a statement on Monday night, claiming the planning was similar in nature to the steps taken before the 4 April sarin gas attack in Khan Sheikhoun that killed dozens of civilians.

The incident prompted US President Donald Trump to launch a “one-off” retaliatory cruise missile strike on a Syrian airbase – the first direct intervention by the US in more than six years of the Syrian civil war.

Michael Fallon says UK will support further action in Syria to stop chemical attacks

“If Mr Assad conducts another mass murder attack using chemical weapons, he and his military will pay a heavy price,” the White House continued.

The Syrian government has consistently denied that it carried out the attack or that it possesses any chemical weapons after giving up its stocks to international inspectors in 2013.

White House officials did not respond to requests for comment on Monday night on the allegations or what form US retaliation would take. Several sources across the US State Department, Pentagon and Central Command, which oversees the country’s military operations in Syria and Iraq – said that they did not know what had prompted the unprecedented threat to the Assad regime.

An official at Central Command told BuzzFeed News they had “no idea” what prompted the statement.

In pictures: US missile strike against Syria Show all 7 1 /7 In pictures: US missile strike against Syria In pictures: US missile strike against Syria The guided-missile destroyer USS Porter (DDG 78) launches a tomahawk land attack missile in the Mediterranean Sea AP In pictures: US missile strike against Syria The United States military launched at least 50 tomahawk cruise missiles at al-Shayrat military airfield near Homs, Syria, in response to the Syrian military's alleged use of chemical weapons in an airstrike in a rebel held area in Idlib province EPA In pictures: US missile strike against Syria Shayrat airfield in Syria Getty Images In pictures: US missile strike against Syria US Navy guided-missile destroyer USS Ross (DDG 71) fires a tomahawk land attack missile in Mediterranean Sea Reuters In pictures: US missile strike against Syria US Navy guided-missile destroyer USS Ross (DDG 71) fires a tomahawk land attack missile in Mediterranean Sea Reuters In pictures: US missile strike against Syria President Donald Trump speaks at Mar-a-Lago in Palm Beach, Fla., after the US fired a barrage of cruise missiles into Syria in retaliation for this week's gruesome chemical weapons attack against civilians AP In pictures: US missile strike against Syria Syria's President Bashar al-Assad Reuters

Several policy advisers at the State Department appeared not to have been briefed before the White House statement went out, learning about the news from reporters instead.

On Tuesday, the White House said it wanted to "clarify that all relevant agencies... were involved in the process from the beginning," spokesperson Sarah Sanders said, noting that the State Department, Pentagon, CIA and Office of the Director of National Intelligence were involved.

"Anonymous leaks to the contrary are false," she added in an emailed statement to Reuters.

There had been “abnormal” activity at an airbase where the Syrian government is suspected of hiding or creating new chemical weapons agents, the Pentagon said on Tuesday, shedding light on the initially unsubstantiated claims.

Although the intelligence was not considered conclusive, the administration issued the swift public warning about the consequences of another chemical attack in an attempt to deter such action, officials said.

Mr Trump himself, who took to Twitter shortly after the statement was published, did not address the threat made to Syria, writing instead about a Fox News report on Barack Obama’s failure to stop Russian interference in the 2016 general election.

The warning to the Syrian government comes three days after US army Colonel Ryan Dillon, spokesperson for the international coalition against Isis, told reporters in Baghdad that the US would welcome concerted efforts from Syria and its Iranian and Russian allies to eliminate the extremists in the east of the country.