The Pentagon said it has detected "active preparations" by Syria for a chemical weapons attack, giving weight to a White House threat that the Syrian government would "pay a heavy price" if it carried out such an atrocity.

The US accusation and ominous warning marked a further escalation of tensions in a country where the US is using Syrian Arab and Kurdish proxy soldiers to combat the Islamic State group in its remaining strongholds, even as Russia and Iran work to support President Bashar Assad, who has gained the upper hand in a long civil war.

A Pentagon spokesman, Navy Captain Jeff Davis, said the US had seen "activity" at Shayrat airfield that "indicated active preparations for chemical weapons use".

That is the same base from which the Syrian air force launched an attack in April that the US and others said used lethal chemicals to kill civilians.

Syria denied the charge.

Nikki Haley, the US ambassador to the United Nations, said the White House warning was meant for a wider audience.

"The goal is at this point not just to send Assad a message, but to send Russia and Iran a message," Ms Haley told a House panel.

"That if this happens again, we are putting you on notice.

"My hope is that the president's warning will certainly get Iran and Russia to take a second look, and I hope that it will caution Assad."

Mr Assad's government and Russia dismissed the White House allegation.

Russian President Vladimir Putin's spokesman Dmitry Peskov said that "such threats to Syria's legitimate leaders are unacceptable".

Russia is Mr Assad's key backer and sided with him when he denied responsibility for the chemical weapons attack that killed dozens of people in Idlib province on April 4.

The US responded to that attack by hitting the airfield with dozens of cruise missiles.

In the days following, Defence Secretary Jim Mattis cautioned that the US was prepared to take further action if Syria repeated such chemical weapons use.

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

Mr Spicer said the activities were similar to preparations taken before the April attack, but he provided no evidence or further explanation.

The statement caught many administration officials by surprise.

Several State Department officials typically involved in co-ordinating such announcements said they were caught off guard by the warning, and it appeared the underlying intelligence information was known only to a small group of senior officials.

Typically, the State Department, the Pentagon and US intelligence agencies would all be consulted before the White House issued a declaration sure to ricochet across foreign capitals.

On Tuesday, deputy White House spokeswoman Sarah Huckabee Sanders said "all relevant agencies", including the Pentagon, State and key intelligence agencies, "were involved in the process from the beginning. Anonymous leaks to the contrary are false."

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 allied forces have faced recent setbacks.

A senior Russian lawmaker dismissed the US warning as "provocation".

Frants Klintsevich, first deputy chairman of the defence and security committee in the upper chamber of the Russian parliament, accused the United States of "preparing a new attack on the positions of Syrian forces".

The US strike in April was the first direct American assault on the Syrian government and President Donald Trump's most dramatic military order since becoming president.

AP

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