Italy has also invited the foreign ministers from several "like-minded countries" - Turkey, Saudi Arabia, United Arab Emirates, Jordan and Qatar - to sit down with the G7 group in the Tuscan city to discuss Syria.
Seated around conference tables in the Tuscan city on Tuesday, the diplomats smiled and exchanged pleasantries but made no remarks as photographers were allowed in briefly for the start of the meeting.
The session lasted roughly an hour.
Calling last week's US missile strike on a Syrian airbase a "game changer", Boris Johnson, UK's foreign minister, said on Monday that support for Assad "was toxifying the reputation of Russia" and suggested that sanctions could be imposed on Russia if it refused to change course.
Rex Tillerson, US secretary of state, is due to travel to Moscow at the end of the two-day gathering with his Italian, German, French, British, Japanese, Canadian and Arab counterparts.
"What we're trying to do is to give Rex Tillerson the clearest possible mandate from us as the West, the UK, all our allies here, to say to the Russians 'This is your choice: stick with that guy, stick with that tyrant, or work with us to find a better solution'," Johnson said after meeting Tillerson.
Johnson said he was keen to seen further sanctions imposed on both Syrian and Russian "military figures".
The diplomatic developments come after a suspected chemical attack on a rebel-held Syrian town that the Syrian Observatory for Human Rights, the UK-based monitoring network, said killed 87 people.
It prompted the US to intervene for the first time directly against the Assad government which it has blamed for the attack.
Tillerson will head to Russia after meeting the ministers from the G7 in Lucca [AFP]
The Syrian government has denied it was behind the April 4 attack in the town of Khan Sheikhoun.
Russia has also rejected the accusations that Assad used chemical arms against his own people while stating that it will not cut its ties with Assad, who has been locked in a six-year-old civil war that has devastated Syria and displaced half its population.
"Returning to pseudo-attempts to resolve the crisis by repeating mantras that Assad must step down cannot help sort things out," Dmitry Peskov, spokesperson for Russian President Vladimir Putin, said on Monday.
For their part, Theresa May, UK prime minister, and US President Donald Trump agreed in a phone call on Monday that a "window of opportunity" exists to convince Russia to end its support for the Syrian president.
"The prime minister and the president agreed that a window of opportunity now exists in which to persuade Russia that its alliance with Assad is no longer in its strategic interest," a spokeswoman for May's Downing Street office said.
Britain said it "fully supported" the US strikes.
In their phone call, Trump and May said Tillerson's Moscow visit, which begins on Tuesday, "provides an opportunity to make progress towards a solution which will deliver a lasting political settlement".
Also on Monday, Justin Trudeau, Canada's prime minister, said in France that his country is ready to stiffen sanctions on Russia.
Source: News agencies