The tweet drew a quick response from Russia’s Foreign Ministry. Spokeswoman Maria Zakharova said on Facebook that “smart” missiles would destroy any evidence of a suspected chemical weapons attack.

The term “smart” missile dates to the introduction decades ago of weapons with advanced guidance systems – for example, using GPS – for precision targeting. Virtually all US missiles have such systems.

Syria also quickly hit back on Wednesday, describing Trump’s threats as a “reckless escalation”, state news agency SANA said.

“We are not surprised by such a reckless escalation from a regime like the United States which has fostered and continues to foster terrorism in Syria,” SANA reported an official source as saying.

Russian lawmakers earlier warned the US that Moscow would view an air strike on Syria as a war crime that could trigger a direct military clash between the two former cold war adversaries. Russia’s ambassador to Lebanon said any missiles fired would be shot down and the launching sites targeted – a stark warning of a potential major confrontation in Syria.

Watch: US, UN respond to Syria chemical gas attack

But Trump’s tweets on Wednesday also delivered a mixed message.

Soon after warning that missiles were coming, Trump tweeted that the US “relationship with Russia is worse now than it has ever been, and that includes the cold war”. Then, striking a more conciliatory tone, he said: “There is no reason for this. Russia needs us to help with their economy, something that would be very easy to do, and we need all nations to work together. Stop the arms race?”

Trump’s administration has sought to show they are being tough on Russia, with a series of economic and diplomatic actions. But the president has also largely avoided criticising Russian President Vladimir Putin by name, although he singled him out in a tweet over the weekend for supporting Syrian President Bashar al-Assad.

Efforts to find a diplomatic solution at the UN Security Council on Tuesday failed, with Washington and Moscow opposing each other’s rival motions to set up an international investigation into chemical weapons use.

The US, France and Britain were in extensive consultations about launching a military strike as early as the end of this week, US officials have said. None of the three countries’ leaders had made a firm decision, according to the officials, who were not authorised to discuss military planning publicly.

President Emmanuel Macron said France, the US and Britain will decide how to respond “in the coming days”. He called for a “strong and joint response” to the attack in the Syrian town of Douma on Saturday, which Syrian activists and rescuers say killed 40 people. The Syrian government denies responsibility.

The French president does not need parliamentary permission to launch a military operation. France is already involved in the US-led coalition created in 2014 to fight Islamic State in Syria and Iraq.

Macron said any action would “target chemical weapons” stocks in Syria.

Trump also spoke by phone with British Prime Minister Theresa May. A British government statement said the two agreed the attack in Syria was “utterly reprehensible” and that the international community must respond “to uphold the worldwide prohibition on the use of chemical weapons.”

Additional reporting by Agence France-Presse