G-7 diplomats gathering in Lucca, Italy, hope to use outrage over the attack and wide international support for the United States' retaliatory missile strikes to push Russia to abandon Assad and join a new peace effort for Syria.
Speaking after meeting with US Secretary of State Rex Tillerson, Johnson said ministers "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".
Last week's nerve gas attack in the rebel-held town of Khan Sheikhoun, which killed more than 80 people, stirred President Donald Trump, who was previously cool to the idea of US intervention, to strike for the first time at Assad's forces. US warships fired 59 cruise missiles at the Syrian air base from which the US believes the attack was launched.
Tillerson said today at the site of a World War II-era Nazi massacre in central Italy that the United States is rededicating itself to hold to account "any and all" who commit crimes against innocent people.
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Alfano said the site of past Nazi atrocities was a reminder that "peace is not a given...That is why we are here to work all together for peace and liberty".
The meeting in the Tuscan walled city of Lucca brings together the foreign ministers of France, Germany, Britain, Japan and Canada, as well as the US and current G-7 president Italy.
Tillerson also spoke by phone with Turkish Foreign Minister Mevlut Cavusoglu, whose government insists Assad should play no role in Syria's future.
Over the weekend, Alfano said that Europe's broad support for the US military strikes had contributed to a "renewed harmony" between the United States and its partners as the G-7 foreign ministers prepared to meet for the first time since Donald Trump took office in January.
"We need to remember that not 10 years ago, but 100 or 120 days ago, the concern in Europe was that the United States and the EU were moving apart," Alfano told Sky TG24 yesterday. "I welcome this renewed harmony."
The G-7 meeting comes as the United States is sending a Navy carrier strike group towards the Korean Peninsula to provide a physical presence following North Korea's persistent ballistic missile tests.
It is also taking place amid an ongoing terror threat that was underscored by the Palm Sunday bombing of Coptic churches in Egypt claimed by the Islamic State group, and another truck attack on European soil, this time in Stockholm, on Friday. The United States is fighting Islamic State group militants in Syria but had previously avoided striking government forces, largely out of concern about being pulled into a military conflict with Russia.
Russia plans to put forward a proposal today for an independent and impartial investigation of the attack, a spokesman for German Foreign Minister Sigmar Gabriel said in Berlin.
The spokesman, Martin Schaefer, said Germany viewed it as "a good and important sign".
Russia was kicked out of the club of industrialised nations, formerly the G-8, after its 2014 annexation of Ukraine's Crimea region and assistance for pro-Russian separatists in eastern Ukraine.
Britain's Johnson, who had been due to visit Russian counterpart Sergey Lavrov in Moscow ahead of today's G-7 meeting, canceled the trip at the last minute, saying the chemical attack had "changed the situation fundamentally".
Tillerson is due to travel to Russia after the G-7 gathering, and Johnson said he will deliver a "clear and coordinated message to the Russians".
Washington has sent mixed signals about whether it shares the determination of allies including Britain that Assad must be removed from power.
After the chemical attack, Trump said his attitude towards Assad "has changed very much" and Tillerson said "steps are underway" to organise a coalition to remove him from power.
Among European nations, there are also differences. While Britain says Assad must go, Alfano was cautious on the issue, saying that the decision should be up to the Syrians.
"I have to say, the Libya experiment did not go well. We are still paying the price," Alfano said, referring to the lawlessness that has ensued since the killing of longtime dictator Muammar Gaddafi and the subsequent flow of migrants to Europe via Italy.