Russia is responsible "by proxy" for the deaths of civilians in a nerve gas attack in Syria last week, the Defence Secretary has said.
Sir Michael Fallon accused Moscow of being complicit in war crimes because it is the Syrian regime's main backer.
A total of 87 people, including many children, died in the suspected sarin attack on the rebel-held town of Khan Sheikhoun in Idlib province on Tuesday.
It prompted President Donald Trump to retaliate by firing 59 cruise missiles at the Syrian airbase where the US believes the gas attack was launched.
Writing in The Sunday Times, Sir Michael added that Russia needed to take action to make sure the "needless suffering" of Syrian civilians can be brought to an end.
He said: "Assad's principal backer is Russia. By proxy Russia is responsible for every civilian death last week.
"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 process."
He said a solution could only come about if Mr Assad quit as president.
This week, the G7 will demand Russia ends its support for Mr Assad and his regime.
Foreign Secretary Boris Johnson has spent the weekend drumming up support for a joint plan to demand Mr Putin pulls his military out of Syria, The Sunday Times reports.
Mr Johnson has scrapped a visit to Russia next week so US Secretary of State Rex Tillerson can go alone and deliver a clear message.
But Russia and Syria's other key ally, Iran, on Saturday vowed to continue fighting in the country after army chiefs General Valery Gerasimov and Major General Mohammad Bagheri spoke by phone.
The two men "condemned the American operation against a Syrian airbase which is an aggression against an independent country", Iran's state news agency IRNA said.
Russia's foreign minister, Sergei Lavrov, has spoken to Mr Tillerson by phone, telling him that the cruise missile attack against the Syrian regime "plays into the hands of extremists".
G7 foreign ministers will meet in Italy on Monday to draw up their coordinated response to Russia's involvement in Syria's civil war.
Mr Tillerson is then expected to travel to Moscow on Tuesday, where he will threaten Vladimir Putin with isolation if he continues to support Mr Assad's regime.
source: sky News
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