Boris Johnson has warned that America could unleash further strikes on Syria in the fight to weaken president Bashar Assad's regime.
The Foreign Secretary was speaking ahead of talks with global counterparts on Monday in an attempt to force Russia to pull out of Syria.
Donald Trump ordered a series of missile strikes last week in response to the deaths of more than 80 people, including children, during a chemical attack in the rebel-held town of Khan Sheikhoun.
Discussing America's response to the attack, something described by Assad's allies as having crossed a "red line", Mr Johnson told The Sun: "Crucially - they could do so again.
"We cannot miss this moment.
"It is time for (Russian president Vladimir) Putin to face the truth about the tyrant he is still propping up."
Mr Putin and Iranian leader Hassan Rouhani said American military intervention was a " violation of international law" during telephone talks backing the Syrian regime.
An arm of Lebanese militant group Hezbollah issued a statement it claimed was on behalf of Russia, Iran and its allies, vowing to "reply with force" to any future aggression "in a variety of ways".
The US and UK have blamed Russia for being complicit in the horrific deaths of innocent Syrians in last week's chemical attack and the federation is facing the threat of fresh international sanctions over its support for the dictator.
Mr Johnson spent the weekend on the telephone to G7 foreign ministers ahead of their two-day meeting in Tuscany and also spoke to key regional players, including the Saudis.
During talks in Italy, he will say that Assad has no future in Syria, Russia must stop supporting the regime and a plan to rebuild the country must be drawn up.
Mr Johnson has been lambasted for pulling out of planned talks with Russian counterpart Sergey Lavrov following discussions with the US.
Critics claimed the move left him looking "daft" and as though he "can't be trusted".
US secretary of state Rex Tillerson will go ahead with his visit this week to deliver a "clear and co-ordinated" message to the Kremlin.
A UK government source said: "The important thing is that this is Britain helping to influence US policy on Syria and Russia , far from being a poodle.
"Three months ago Syria wasn't really an issue for them but our push and recent events have made a difference."
Defence Secretary Sir Michael Fallon said Russia was responsible for "every civilian death" in the Syrian chemical weapons attack, claiming President Putin was to blame "by proxy" as Assad's "principal backer".
Mr Tillerson echoed the comments, telling Face The Nation on CBS, the Russians "have played now for some time the role of providing cover for Bashar Assad's behaviour".
Russia's failures to rid Syria of weapons stockpiles "has led to the killing of more children and innocents", he said.
But asked about the possibility of further intervention, the secretary of state said Washington's "first priority" in Syria was to defeat the Islamic State terror group, also known as Daesh.
"Once the Isis threat has been reduced or eliminated, I think we can turn our attention directly to stabilising the situation in Syria," he said.
Some 87 people, including children, are believed to have been killed in the suspected sarin nerve agent strike on Khan Sheikhoun.
Mr Trump subsequently ordered a strike by 59 cruise missiles on the base in America's first direct attack against the Syrian government.