Russian and Iranian forces last night warned Donald Trump they would retaliate with military action if he launched more airstrikes on Syria.
In an ominous threat raising the prospect of war, they said the US President had crossed a
"red line" with his surprise missile bombardment on Bashar al-Assad's forces.
"From now on we will respond with force to any breach of red lines and America knows our ability to respond well," the military chiefs said in a joint statement with Hezbollah.
The Russian Embassy in London last night suggested there could be "real war" if Moscow is presented with an ultimatum over Syria, the Daily Mail reports.
But Boris Johnson will today lead a push for Russia to face tough new sanctions unless it withdraws its support for Assad.
At a G7 meeting in Italy, the Foreign Secretary will call for Moscow to be threatened with isolation from the international community and a raft of economic punishments.
On a day of escalating tensions yesterday:
• The Kremlin mocked Britain for having ''no real influence on the course of international affairs'' after Mr Johnson cancelled a planned trip to Moscow;
• Defence Secretary Sir Michael Fallon said Russia was responsible for ''every civilian death'' in Assad's chemical weapons attack on his own people;
• America warned Moscow it faces a ''relationship of competition and potential conflict'' unless it stops supporting Assad's ''murderous regime'';
• The White House told North Korea it should take US missile strikes against Syria as a warning that America is prepared to take military action against countries which break international agreements.
In the early hours of Friday, President Trump ordered airstrikes on a Syrian airbase in the first direct attack by the US against Assad.
The onslaught of 59 cruise missiles followed a suspected chemical attack on the rebel-held town of Khan Sheikhun in northern Syria which killed 87 civilians, including many children.
A joint command centre of Russia, Iran and Islamist militant group Hezbollah on Sunday warned America they would not allow further attacks on the Syrian regime.
They said: "What America waged in an aggression on Syria is a crossing of red lines. From now on we will respond with force to any aggressor or any breach of red lines from whoever it is and America knows our ability to respond well."
The phrasing appeared to be a mocking reference to Barack Obama's warning in 2012 that Assad should not cross a ''red line'' by using chemical weapons.
In a further sign of increasing tensions, the Russian Embassy in London responded with anger to claims the G7 - Canada, France, Germany, Italy, Japan, Britain and America - could today agree to slap Putin with the threat of sanctions unless he pulls his troops out of Syria and ends his support for Assad.
In a series of provocative tweets on its official account, the embassy suggested that if Russia were given an ultimatum, the outcome would be either, "war of clowns, war of muses, a conventional war or a mix of the above".
It added: "If G7 ultimatum to Russia brings us to real war, what is your trust in @realDonaldTrump as a wartime leader & @BorisJohnson as his lieutenant?" Iran's supreme leader, Ayatollah Ali Khamenei, last night said the US had made a 'strategic mistake' by attacking Syria."
He made clear he was not intimidated by claims the airstrike was a warning that America was willing to target other countries. Tehran has been a key backer of Assad in Syria's six-year civil war.
Khamenei said: "The Islamic Republic of Iran will not leave the field in the face of threats. Former American officials created Daesh [Islamic State] and the current leaders are reinforcing it"
Trump administration officials yesterday blamed Russian inaction for enabling the deadly poison gas attack against Syrian civilians last week. US Secretary of State Rex Tillerson said Syria was able to execute the attack, which killed scores of people, because Moscow had failed to carry out a 2013 agreement to secure and destroy chemical weapons in the country.
White House national security adviser HR McMaster said Syria's "sponsors", Russia and Iran, were enabling Assad's ''campaign of mass murder against his own civilians''.
He said the US would take further action in Syria if necessary.
"We're prepared to do more. The president will make whatever decision he thinks is in the best interests of the American people," he told Fox News.
Mr McMaster said Moscow was supporting "a murderous regime" and its actions would dictate the future of US-Russian relations.
"Do they want it to be a relationship of competition and potential conflict," he said. 'Or do they want it to be a relationship in which we can find areas of co-operation that are in our mutual interest?'
Last night a US strike group led by an aircraft carrier steamed toward the Korean peninsula as US Secretary of State Rex Tillerson said the strikes against Syria carried a message for any nation operating outside of international norms.
"If you violate international agreements ... if you become a threat to others, at some point a response is likely to be undertaken," he told ABC TV.
Mr McMaster said it was prudent to send the strike group, criticising North Korea as a rogue, nuclear-armed nation engaged in provocative behaviour.