Russia and Iran have threatened military retaliation against the US, accusing US President Donald Trump of crossing “red lines” by ordering a cruise missile attack on a Syrian air base.

The two military allies of Syria said on Sunday night the US bombardment had violated international law and, in a statement, added: “From now on we will respond with force.”

The warning came after the Russian embassy in Britain suggested that British and American attempts to deliver an ultimatum to the Kremlin this week could result in a “real war”.

Boris Johnson, the British Foreign Secretary, is understood to be working on a proposal from the G7 group of nations which will demand that Russian President Vladimir Putin remove his troops from Syria and drop his backing for Syrian leader Bashar al-Assad.

Johnson cancelled plans to visit Moscow this week to work on the proposal, which the Telegraph understands will include a tacit offer to Russia to rejoin the G7 if it complies.

On Sunday, Russia mocked Johnson, saying his refusal to visit was “deplorable” and, in a series of gibes on Twitter, questioned whether he would make a fit wartime lieutenant to the US President.

The threat from Russian and Iranian forces came as Washington intensified pressure on Assad and his backers, with Trump’s administration saying there was now “no way” a peace settlement could be found with the Syrian dictator in power.

In a significant change of stance towards regime change in Syria, Nikki Haley, the US ambassador to the UN, said last week’s missile strikes were designed to let Russia know, “We’re not going to have you cover for this regime any more.”

H.R. McMaster, White House national security adviser, said Russia and Iran were enabling Assad’s “campaign of mass murder against his own civilians” and the US was “prepared to do more”.

Rex Tillerson, US Secretary of State, said Washington would expect Russia to rethink its support for Assad because “every time one of these horrific attacks occurs, it draws Russia closer into some level of responsibility”.

However, a joint command centre made up of forces from Russia, Iran and militias supporting Assad said they would redouble their backing, after 59 US cruise missiles hit Shayrat air base in retaliation for a suspected chemical attack on the rebel-held town of Khan Sheikhun.

A statement from the centre 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 joint command centre also said the presence of US troops in northern Syria where Washington has hundreds of special forces helping the Syrian Democratic Forces to oust IS was “illegal” and that Washington had a long-term plan to occupy the area.

In a separate phone call between Moscow and Tehran, Putin and Iran’s President Hassan Rouhani said that US aggression against Syria violated international law.

The two leaders also called for an investigation into the nerve attack in Idlib province, which Syria has claimed was caused by a stockpile of rebel-held chemical weapons being hit by a stray bomb, and said they were ready to deepen co-operation to fight terrorism.

Ayatollah Ali Khamenei, Iran’s supreme leader, also accused the US of making a “strategic mistake” by attacking Syria and said the country would not back down.

Alex Salmond, the Scottish National Party’s foreign affairs spokesman, said Johnson looked “daft” for pulling out of a meeting with the Russians in Moscow.

He told the Andrew Marr Show: “What is the argument for not going ahead with a visit?

“Rex Tillerson is going on Wednesday so it can’t be that we have moved to a Cold War position of no talking whatsoever.

“The idea the Foreign Secretary can’t be trusted because he might pursue his own line or have an independent thought or crossover what the Americans are going to say just makes him look like some sort of mini-me to the United States of America.”