Russia is to blame for “every civilian death” in last week’s chemical weapons attack in Syria, Defence Secretary Sir Michael Fallon has claimed.
Sir Michael, writing in the Sunday Times, said the Kremlin was responsible “by proxy” as the “principal backer” of President Bashar al-Assad’s regime.
It comes after Foreign Secretary Boris Johnson cancelled a visit to meet his opposite number in Moscow.
The attack has been widely blamed on the Syrian government.
Sir Michael said last Tuesday’s gas attack was “barbaric, immoral and illegal” and the response of air strikes from US President Donald Trump was “the right call”.
Referring to Russia, Sir Michael wrote: “This latest war crime happened on their watch.
“In the past few years, they have had every opportunity to pull levers and stop this civil war. Russia must show the resolve necessary to bring this regime to heel.”
The defence secretary said Syria needed a government in which Mr Assad played no part. He acknowledged achieving that was “not easy, but not impossible”.
“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 future,” said Sir Michael.
“Justified and appropriate though US action was, it was also necessarily limited. We now need a long-term solution to this conflict.”
Sir Michael said Mr Assad must depart and the search for stability begin.
“Someone who uses barrel bombs and chemicals to kill his own people simply cannot be the future leader of Syria,” he stated.
Shadow Foreign Secretary Emily Thornberry said Labour’s official position was that the Trump administration should not have acted without UN backing but she conceded that senior figures, including deputy leader Tom Watson, took a different view.
Mr Johnson, who called off the trip to Moscow on Monday, is attempting to co-ordinate a united response from the G7 group of industrialised nations, whose foreign ministers meet in Italy this week.
The Russian foreign ministry said Mr Johnson’s decision not to visit Russia was “absurd”.
US Secretary of State Rex Tillerson will visit Moscow as planned after the G7 meeting in Italy this week.
Eighty-nine people, including 33 children and 18 women, died in the suspected nerve agent attack in the rebel-held town of Khan Sheikhoun on Tuesday.
The figures come from Idlib’s opposition-run health authority. The country’s government denies using nerve gas.
On Friday, the US carried out missile strikes on a Syrian air base it says is suspected of storing chemical weapons. At least six people are reported to have died.
In response, Syria’s ally Russia accused the US of encouraging “terrorists” with unilateral actions.
Moscow has further promised to strengthen Syria’s anti-aircraft defences and it is shutting down a hotline with the US designed to avoid collisions between their air forces over the country.
Iran’s President Hassan Rouhani has said “terrorists are celebrating” the US strikes on the air base.
But he also backed calls for an independent inquiry into the suspected chemical weapons attack.
Mr Johnson’s visit, for talks with his Russian counterpart Sergey Lavrov, would have been the first by a UK foreign secretary in more than five years.
Critics have claimed Mr Johnson is in political trouble after cancelling the meeting.
The SNP’s Foreign Affairs spokesman Alex Salmond said: “Boris Johnson just looks daft.
“I mean, what is the argument for not going ahead with a visit?
“Rex Tillerson is going in Wednesday so it can’t be that we have moved to a Cold War position of no talking whatsoever.
“And the idea that the foreign secretary can’t be trusted because he might pursue his own line or have an independent thought or cross over what the Americans might say makes him look like some kind of ‘mini-me’ to the USA and that’s not a position that any foreign secretary would want to be in.”
Liberal Democrat leader Tim Farron claimed Mr Johnson was considered a diplomatic liability.
“Is this what taking back control looks like? Our government quick to blindly follow every order from the Trump White House,” said Mr Farron.
“Boris has revealed himself to be a poodle of Washington, having his diary managed from across the pond.
“It is pretty shameful when even Trump judges you to be a buffoon.”
An aide to Mr Johnson said: “It is a shame the Lib Dems would rather snipe and be silly when the US and UK are trying to work on a plan to help the innocent people of Syria and stop a devastating civil war.”
Asked by the BBC’s Andrew Marr if Mr Johnson had been ordered to cancel his visit to Russia by the Americans, International Development Secretary Priti Patel said: “This isn’t just about one voice.
“This is about the international community coming together and our foreign secretary is working with his American counterpart as that is the right thing to do.”