The US could launch further strikes against Assad regime forces in Syria, Boris Johnson has said, hours after announcing that he would cancel a long-planned trip to Russia.

The Foreign Secretary was speaking ahead of G7 international talks scheduled for Monday and aimed at agreeing a common position on the Syrian civil war.

Issues on the table at the meeting will include the possibility of forcing Russian withdrawal from the conflict, which Vladimir Putin has been intervening in on the side of the Syrian government.

That meeting comes ahead of a visit to Russia by US secretary of state Rex Tillerson. Mr Tillerson says he will deliver a “clear and co-ordinated” message to the Kremlin about its role in the civil war.

Mr Johnson was criticised for cancelling the meeting of his own to make way for Mr Tillerson. The Foreign Secretary was branded a “poodle” by both the Kremlin and other British politicians, for having his “diary managed from across the pond”.

US President Donald Trump ordered missile strikes against Syrian government military targets last week in response to the deaths of more than 80 people during an alleged chemical attack in the rebel-held town of Khan Sheikhoun.

Discussing the round of missile strikes, Mr Johnson told The Sun newspaper: “Crucially - they could do so again. We cannot miss this moment.

“It is time for [Russian president Vladimir] Putin to face the truth about the tyrant he is still propping up.”

The US and UK blame Russia for being complicit in last week's chemical attack and the ongoing brutal conduct of the Syrian government. Defence Secretary Sir Michael Fallon on Sunday claimed that Russia was responsible for "every civilian death" in the Syrian chemical weapons attack, because it was the Syrian government’s “principal backer”.

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.

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

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

Donald Trump ordered missile strikes against US president Assad (JIM WATSON/AFP/Getty Images)

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

The source did not explain that influence the UK has so far had on US policy.

Some 87 people, including children, are believed to have been killed in the suspected sarin nerve agent strike on Khan Sheikhoun.

In pictures: US missile strike against Syria Show all 7 1 /7 In pictures: US missile strike against Syria In pictures: US missile strike against Syria The guided-missile destroyer USS Porter (DDG 78) launches a tomahawk land attack missile in the Mediterranean Sea AP In pictures: US missile strike against Syria The United States military launched at least 50 tomahawk cruise missiles at al-Shayrat military airfield near Homs, Syria, in response to the Syrian military's alleged use of chemical weapons in an airstrike in a rebel held area in Idlib province EPA In pictures: US missile strike against Syria Shayrat airfield in Syria Getty Images In pictures: US missile strike against Syria US Navy guided-missile destroyer USS Ross (DDG 71) fires a tomahawk land attack missile in Mediterranean Sea Reuters In pictures: US missile strike against Syria US Navy guided-missile destroyer USS Ross (DDG 71) fires a tomahawk land attack missile in Mediterranean Sea Reuters In pictures: US missile strike against Syria President Donald Trump speaks at Mar-a-Lago in Palm Beach, Fla., after the US fired a barrage of cruise missiles into Syria in retaliation for this week's gruesome chemical weapons attack against civilians AP In pictures: US missile strike against Syria Syria's President Bashar al-Assad Reuters

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

The base appears to be at least partly operational despite the strike, however, with reports of warplanes taking off from it in the days following the attack.