The US said on Tuesday that it had observed preparations for a possible chemical weapons attack at a Syrian air base allegedly involved in a sarin attack in April following a warning from the White House that the Syrian regime would “pay a heavy price” for further use of the weapons.

Reporters traveling in Germany with the US defence secretary, James Mattis, were told that the Pentagon was prepared to take action after activity was seen at the Shayrat base similar to the pattern that preceded the April gas attack on Khan Sheikhoun, which killed at least 80 people. That incident prompted a US missile strike on the base, although the strike did not seriously impair its operations.

Pentagon spokesman Cpt Jeff Davis said that the activity at the base had taken place in “the past day or two”.

“This involved specific aircraft in a specific hangar, both of which we know to be associated with chemical weapons use,” he told Reuters.

In a bluntly worded statement released late on Monday night, the White House said: “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 activities are similar to preparations the regime made before its April 4, 2017 chemical weapons attack,” the White House said. “As we have previously stated, the United States is in Syria to eliminate the Islamic State of Iraq and Syria. If, however, Mr Assad conducts another mass murder attack using chemical weapons, he and his military will pay a heavy price.”

The unusual public warning on Monday night appeared to be intended to deter the regime from repeating its use of chemical weapons against rebel-held cities and towns.



It may also have been aimed at the regime’s backers in Moscow and Tehran, who have resolutely backed Assad and denied the regime’s responsibility for chemical weapons use.

The French presidency said in a statement on Tuesday that Emmanuel Macron and Donald Trump had agreed during a telephone call on the need for a “joint response” in the event of another chemical attack in Syria but refused to say if it had evidence of one under preparation.

After a meeting last month with Vladimir Putin, an Assad ally, Macron drew a “very clear red line” on the use of chemical weapons “by whomever” and warned of reprisals.



The Kremlin on Tuesday described the US warning as an unacceptable threat and said Russia had no information about a new chemical attack. However, the Russian and Syrian militaries are closely intertwined. The White House warning came on the same day the chief of the Russian general staff, General Valery Gerasimov met President Bashar Assad at Khmeimim air base near Latakia.

Mattis did not address the nature of the intelligence or White House warning as he flew to Europe for a Nato meeting, but said the US was not going to get pulled into the conflict between the regime and the armed opposition.

“We just refuse to get drawn into a fight there in the Syria civil war, we try to end that one through diplomatic engagement,” he told reporters. “If somebody comes after us, bombs us or takes a heading on us or fires on us, then under legitimate self-defence we’ll do whatever we have to do to stop it.”

He said the US would not fire “unless they are the enemy, unless they are Isis. He did not mention the administration’s response to the regime’s use of chemical weapons.

Although the focus of US military operations in the region is the defeat of Isis in its two major strongholds, Mosul in Iraq and Raqqa in Syria, the Trump administration has showed itself willing to act if the Assad regime carries out a major chemical weapons attack.



On 6 April, Donald Trump ordered a salvo of 59 Tomahawk missiles against Shayrat base in response to the sarin attack on Khan Sheikhoun. Russians were present at the Shayrat base, near Homs, but US planners ensured that the missiles used in the night-time strike fell well away from the Russian compound.

“President Trump’s warning to the Assad regime not to use chemical weapons against Syrians is based on intelligence submitted to the administration about such [a] possibility,” Walid Phares, a Trump adviser on the Middle East during the campaign, told the Guardian.

Phares suggested the main intended recipient of the White House message was Moscow.

“The situation is delicate as Russia warned [against] US air action over Assad regime assets,” he said. “Hence it is maybe up to the Russians to insure that Assad won’t use these weapons so that no escalation in Syrian airspace could happen.”

The UK’s defence secretary, Michael Fallon, said the US had not shared any evidence of a specific threat of a chemical weapons attack.



“We are very clear the use of chemical weapons by the Syrian regime is absolutely abhorrent and the last time the United States took action to deal with the aircraft and airbase from which these chemical weapons were used we fully supported their strike,” Fallon told the BBC.

The UK would support any US attack on Assad so long as it was proportionate, legal and necessary, Fallon added.

“The White House must have solid intelligence about a possible Syrian sarin attack but why they chose to send [a] message to Assad and Putin via press release isn’t clear,” Daryl Kimball, the head of the Arms Control Association said in a tweet. Kimball said that the risk of coming into conflict with Russian forces in the event of another punitive US strike was higher now than it was in April.

Frants Klintsevich, who sits on a defence committee in the Russian parliament, criticised the White House warning. The US is “preparing a new attack on the positions of Syrian forces”, Klintsevich told state-owned RIA Novosti, adding: “Preparations for a new cynical and unprecedented provocation are underway.”

US forces in Syria have also been empowered to defend themselves and their allies against attack, which has led to a string of recent clashes with pro-regime forces competing for the same territory.