Russia is responsible for “every civilian death” in Bashar Assad’s chemical weapons attack on his own people, the Defence Secretary has said.
Sir Michael Fallon said Vladimir Putin’s Kremlin was to blame “by proxy” because it was the Syrian president’s “principal backer”.
His comments came after Foreign Secretary Boris Johnson pulled out of a Moscow visit hours before he was due to fly.
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.
US president Donald Trump ordered a strike by 59 cruise missiles on the base he said launched the attack in America’s first direct attack against the Syrian government.
Writing in the Sunday Times, Sir Michael said Russia must be part of the solution to ending the “needless suffering” of Syrian civilians.
“But 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.”
He also reiterated his support of Mr Trump’s cruise missile strike in the early hours of Friday, UK time.
“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,” he wrote.
But there must now be a long-term solution to the civil war in which Assad must no longer be president, he added.
“Someone who uses barrel bombs and chemicals to kill his own people simply cannot be the future leader of Syria,” he said.
Mr Johnson is to spearhead demands for Russia to withdraw its forces from Syria when he meets G7 leaders in Italy next week.
He said the UK continues to “deplore” Moscow’s defence of Assad’s regime as he pulled out of talks with Russian counterpart Sergey Lavrov.
But the cancellation, which came after discussions with US secretary of state Rex Tillerson, led to him being criticised as “Washington’s poodle”.
Mr Tillerson will go ahead with his visit to Russia to “deliver that clear and co-ordinated” message to Moscow, which continues to deny Syrian forces used chemical weapons last week.
International Development Secretary Priti Patel refused to say if Britain would support future strikes by the US if there are fresh chemical attacks in Syria.
She told BBC One’s Andrew Marr Show: “We are not going to speculate on hypothetical scenarios and situations but what we will do is continue to provide the life saving support that is required when we see such enormous medical emergencies.”
Shadow foreign secretary Emily Thornberry said “continuing to bomb in Syria is not the solution”.
She told Marr: “We have to be part of international agreement, so if the international community felt that the only way in which this could be dealt with would be by way of military action...
“The question is how do we actually bring this war to an end? That’s where we actually start from.”
Pressed on under what circumstances Labour would support a strike against Assad, she replied: “If there is a plan. There has to be a plan.”