Washington: The White House said late on Monday that Syrian President Bashar al-Assad  appeared to be preparing another chemical weapons attack, and warned that he would "pay a heavy price" if one took place.

Several military officials were caught off-guard by the statement from US President Donald Trump's press secretary, but it was unclear how closely held the intelligence regarding a potential chemical attack was.

In the statement, the White House said Assad's preparations appeared similar to the ones Western intelligence officials believe the Syrian government made before a chemical attack in April that killed dozens of Syrians, including children.

"As we have previously stated, the United States is in Syria to eliminate the Islamic State of Iraq and Syria," the statement said. "If, however, Mr Assad conducts another mass-murder attack using chemical weapons, he and his military will pay a heavy price."

While the White House's motivation in releasing the highly unusual statement is uncertain, it is possible that Trump or his advisers decided a public warning to Assad might deter another chemical strike.

Any intelligence gathered by the United States or its allies - notably Israel, which keeps a robust watch on unconventional weapons in the Middle East - would by nature be classified. But any US president has absolute power to declassify anything they choose to release.

Brian Hale, a spokesman for the director of national intelligence, referred questions to the White House. Marc Raimondi, a spokesman for the White House's National Security Council, said: "We are letting the statement speak for itself."

Nikki Haley, the US ambassador to the United Nations, made clear that the United States was taking the latest threat seriously.

"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 said in a Twitter post.

Russia and Iran are both allied with the Assad government. Last week, after the US downed a Syrian warplane that had dropped bombs near US-supported fighters battling the Islamic State, Russia's Defence Ministry threatened to target any aircraft flown by the US or its allies west of the Euphrates River valley.

That statement led to a suspension of RAAF operations in Syrian airspace.

Daryl Kimball, the executive director of the Arms Control Association, said that he had not heard of Syrian moves toward more chemical attacks, but that he suspected intelligence reports had prompted the statement. Rocket attacks using sarin gas, as in the April strikes, require considerable preparation that US intelligence might well have picked up, he said.

Kimball added that he did not recall such a precise, pre-emptive public warning against a foreign government regarding banned weapons "in at least the last 20 years". More often, such matters are handled in private diplomatic or intelligence communications, he said.

Monday's message appeared designed to set the stage for another possible military strike. After Assad allegedly used chemical weapons in April, the US military fired 59 Tomahawk cruise missiles at the air base his government had used to launch the attack.

The use of chemical weapons by the Syrian government has long been part of the clash between Assad and the US.

In 2013, president Barack Obama's intelligence agencies concluded with "high confidence" that Assad had carried out a devastating chemical attack that killed hundreds of Syrians, despite having been warned by Obama against crossing a "red line" by using chemical weapons.

But Obama stopped just short of ordering a military strike, instead opting to work with the Russian government to identify and destroy Assad's cache of chemical weapons. Critics argued that the president's failure to enforce his own "red line" had emboldened Assad.

They also warned that all the chemical weapons could not be found and destroyed. Within two months of Trump's taking office, images of another chemical attack spurred him to take the action that Obama had rejected.

New York Times