The United States has been warned it crossed a “red line” with its missile strikes on Syria and faces retaliation in response to future aggression against Bashar Assad.

Russian president Vladimir Putin and Iranian leader Hassan Rouhani said American military intervention was a “violation of international law” during telephone talks backing the Syrian regime.

An arm of Lebanese militant group Hezbollah issued a statement it claimed was on behalf of Russia, Iran and its allies, vowing to “reply with force” to any future aggression “in a variety of ways”.

Boris Johnson will meet counterparts from leading nations in Italy for talks on Monday that will focus on pressuring Moscow to pull out of Syria and ending Assad’s grip on power.

The US and UK have blamed Russia for being complicit in the horrific deaths of innocent Syrians in last week’s chemical attack and the federation is facing the threat of fresh international sanctions over its support for the dictator.

Mr Johnson spent the weekend on the telephone to G7 foreign ministers ahead of their two-day meeting in Tuscany and also spoke to key regional players, including the Saudis.

He will say that Assad has no future in Syria, Russia must stop supporting the regime and a plan to rebuild the country must be drawn up.

USS Porter firing a tomahawk land attack missile from the Mediterranean Sea
USS Porter firing a Tomahawk land attack missile from the Mediterranean Sea (US Navy/PA)

Mr Johnson told The Sun US president Donald Trump had shown “a resolve and willingness that has been sadly missing in the last few years” and warned the US could strike Syria again.

He added: “We cannot miss this moment. It is time for Putin to face the truth about the tyrant he is still propping up.”

Mr Johnson has been lambasted for pulling out of planned talks with Russian counterpart Sergey Lavrov following discussions with the US.

Critics claimed the move left him looking “daft” and as though he “can’t be trusted”.

US secretary of state Rex Tillerson will go ahead with his visit this week to deliver a “clear and co-ordinated” message to the Kremlin.

A UK government source said: “The important thing is that this is Britain helping to influence US policy on Syria and Russia, far from being a poodle.

“Three months ago Syria wasn’t really an issue for them but our push and recent events have made a difference.”

Defence Secretary Sir Michael Fallon said Russia was responsible for “every civilian death” in the Syrian chemical weapons attack, claiming President Putin was to blame “by proxy” as Assad’s “principal backer”.

Mr Tillerson echoed the comments, telling Face The Nation on CBS, the Russians “have played now for some time the role of providing cover for Bashar Assad’s behaviour”.

Russia’s failures to rid Syria of weapons stockpiles “has led to the killing of more children and innocents”, he said.

But asked about the possibility of further intervention, the secretary of state said Washington’s “first priority” in Syria was to defeat Islamic State terror group, also known as Daesh.

Some 87 people, including children, are believed to have been killed in a suspected sarin nerve agent strike on the rebel-held town of Khan Sheikhoun.

Mr Trump ordered a strike by 59 cruise missiles on the base in America’s first direct attack against the Syrian government.