Senior Russian military officers could face international sanctions for assisting Bashar al Assad's campaign against his own people, Foreign Secretary Boris Johnson has said.
Speaking after a meeting with US Secretary of State Rex Tillerson, Mr Johnson said Moscow must now decide whether to stick with the "toxic" Syrian president or work with the G7 nations to find a political solution to the conflict in Syria.
"Do they want to stick with a toxic regime? Do they want to be eternally associated with a guy who gases his own people?" he asked.
"Or do they want to work with the Americans and the rest of the G7 and indeed like-minded countries for a new future for Syria?"
Foreign ministers from the G7 nations have gathered in Lucca, Italy to consider a response to a chemical attack in Idlib province which killed more than 80 people, including many children.
"We will be discussing the possibility of further sanctions certainly on some of the Syrian military figures and indeed on some of the Russian military figures who have been involved in co-ordinating the Syrian military efforts and are thereby contaminated by the appalling behaviour of the Assad regime," Mr Johnson told reporters.
He did not comment on reports the US and UK are ready to offer Moscow the chance to rejoin the G8 if it drops its support for Mr Assad.
Russia has stood by the Syrian regime following the atrocity, claiming that Assad's military hit a warehouse used by the rebels to store chemical weapons.
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It has also criticised the American decision to hit a Syrian airbase with 59 cruise missiles in the aftermath of the attack.
Mr Putin and his Iranian counterpart, Hassan Rouhani, said on Monday that the strikes had crossed "red lines" - echoing US President Donald Trump's condemnation of the gas attack itself.
Italian foreign minister Angelino Alfano, who is hosting the talks, discussed the situation in Syria in a telephone phone call with his Iranian counterpart.
He urged Tehran to use its influence with the Syrian regime "to avoid new attacks, completely eliminate chemical weapons and assure a ceasefire".
Earlier, as he visited the the site of a Nazi atrocity at Santa'Anna di Stazzema, where 560 people were killed in 1944,
Mr Tillerson said the Washington would hold to account "any and all" who commit crimes against civilians.
He is expected to deliver a "clear and coordinated" message to the Kremlin later this week after Mr Johnson cancelled his own visit to the Russian capital following conversations with Mr Trump's team.
Mr Putin's spokesman Dmitry Peskov added that Mr Tillerson would be meeting his Russian counterpart Sergey Lavrov and not the Russian president.