The White House has warned the Syrian regime that it will pay a “heavy price” for any chemical weapons attack after Washington said it had detected preparations for such an operation.

Two months after President Donald Trump responded to a Syrian chemical weapons attack by launching 59 Tomahawk missiles at the country, the White House said US officials had identified possible preparations for a similar attack by the regime of Bashar al-Assad.

“The US 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,” Sean Spicer, White House press secretary, said in a statement.

Mr Spicer said the Syrian actions were similar to activity that preceded the April 4 chemical weapons attack that sparked the first major military operation under the Trump presidency.

“As we have previously stated, the US is in Syria to eliminate the Islamic State of Iraq and Syria [Isis],” Mr Spicer added. “If, however, Mr Assad conducts another mass murder attack using chemical weapons, he and his military will pay a heavy price.”

The White House did not provide further information, but it appeared that the highly unusual public warning was designed to change the calculus of the Syrian regime. Asked if the Pentagon was preparing to strike Syria, a spokesman said: “I have nothing.”

The warning was also unusual for the White House given that Mr Trump frequently argues that the US should not telegraph any potential military operations.

At the same time, the president has recently attempted to signal a more aggressive stance overseas than his predecessor Barack Obama, who faced criticism from even within his own cabinet for failing to live up to a threat to attack Mr Assad after he used chemical weapons in 2013.

Tensions have increased in recent weeks as the Syrian conflict threatens to become a flashpoint for a broader international confrontation, with Russia angrily denouncing the US’s downing of a Syrian fighter jet last week and threatening to target US-led coalition aircraft. Iran also fired missiles into the country to target Isis in what was seen as a deliberate display of force.

Late on Monday Nikki Haley, the US ambassador to the UN, said that Russia and Iran, which have intervened militarily to back the Assad regime in the six-year conflict, would also be held responsible for any new chemical weapons attack.

“Any further attacks done to the people of Syria will be blamed on Assad, but also on Russia and Iran who support him killing his own people,” she tweeted.

Michael Fallon, the UK defence secretary, said Britain would support any US action against Syria to prevent chemical weapons attacks.

Sir Michael told the BBC that the UK had not seen the intelligence on which the White House was basing its threat, but said the government would support US action.

“We’re very clear what our position is. The use of chemical weapons by Syria is absolutely abhorrent,” he said.

After Mr Trump ordered the launch of the Tomahawks in April, he defended the missile strikes as having been “in the vital security interest of the US” to prevent the spread or use of chemical weapons. The attack followed debate in the White House about whether Mr Trump, who campaigned on an isolationist foreign policy, should take military action against the regime.

If . . . Mr Assad conducts another mass murder attack using chemical weapons, he and his military will pay a heavy price

Some of the president’s advisers, including his daughter Ivanka Trump, had argued that he should not tolerate a gas attack that had killed more than 70 people, including young children. They ultimately prevailed over others, including Steve Bannon, the top White House strategist, who had argued that Mr Trump should focus on domestic issues and not get involved in the Syria conflict.

After ordering the missile strike, Mr Trump said the chemical attacks were “an affront to humanity” and “crossed a lot of lines”.

During last year’s US presidential campaign, Mr Trump criticised Mr Obama for setting a “red line” over the use of chemical weapons in Syria but not responding when it was crossed. But he also criticised his predecessor for later considering military strikes on the Assad regime.

Bouthaina Shaaban, a senior adviser to Mr Assad, last week told the Financial Times that the US was “waging a war of attrition” against the regime.

“Their intention is not to end the Syrian conflict, but it is like a war of attrition against the Syrian people and against the Syrian army,” she said.

Russia and Iran’s intervention has tilted the war in the Assad regime’s favour. Much of the fighting between the rebels and government forces has subsided since Moscow, Tehran and Ankara, which supports the opposition, agreed a deal to establish “de-escalation zones” in the main battlegrounds.

However, fierce fighting has broken out in the past two weeks between government and rebel forces around Damascus and the southern province of Deraa.

In the south, battles have been raging while Russia and the US have been negotiating over the creation of a safe zone in that area as rival sides try to position ahead of any political deal. There has also been an escalation of clashes between government forces and their allies and