Defence Secretary Sir Michael Fallon has demanded Russia rein in Syrian President Bashar al-Assad, claiming that Moscow is “responsible for every civilian death” in last week’s chemical weapons attack on Khan Sheikhun.

Sir Michael said the attack had happened “on their watch” and that Vladimir Putin must now live up to previously made promises that Mr Assad’s chemical weapons have been destroyed.

The cabinet minister’s intervention comes as the West unleashes a new diplomatic drive on the back of the retaliatory missile strikes launched by Donald Trump’s US administration.

Later this week US Secretary of State Rex Tillerson will travel to Moscow, while Foreign Secretary Boris Johnson will garner support at a G7 summit of foreign ministers.

Writing in The Sunday Times, Sir Michael said Russia must be part of the solution to ending the “needless suffering” of Syrian civilians.

“Assad’s principal backer is Russia. By proxy Russia is responsible for every civilian death last week,” he said.

“If Russia wants to be absolved of responsibility for future attacks, Vladimir Putin needs to enforce commitments, to dismantle Assad’s chemical weapons arsenal for good, and to get fully engaged with the UN peacekeeping progress.”

After previous gas attacks in Syria which the West blamed on the Assad regime, Mr Putin made a deal with former-President Barack Obama to scrap Syria’s arsenal of deadly chemical weapons.

Syria: What led to the US airstrikes?

But Sir Michael said the launching of almost 60 missiles at the regime airbase from which the attack is said to have originated would now make future incidents less likely.

He wrote: “By sending Tomahawk missiles to attack the airfield, aeroplanes and equipment believed to be involved, it has sent a strong signal to the Syrian regime to think twice before using gas in the future.”

Sir Michael went on that there must now be a longer-term solution to the conflict that does not involve Mr Assad, adding: “Someone who uses barrel bombs and chemicals to kill his own people simply cannot be the future leader of Syria.”

Mr Johnson is to spearhead demands for Russia to withdraw its forces from Syria when he meets G7 leaders in Italy this week.

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

He said the UK continues to “deplore” Moscow’s defence of Mr Assad’s regime as he pulled out of talks with Russian counterpart Sergey Lavrov.

But the cancellation of tomorrow’s meeting, which came after discussions with US Secretary of State Rex Tillerson, led to him being criticised as “Washington’s poodle”, an attack dismissed by the Foreign Office.