Britain's Foreign Secretary Boris Johnson set the tone for the meeting, describing Syrian President Bashar al-Assad as "toxic" and saying it was "time for (Russian President) Vladimir Putin to face the truth about the tyrant he is propping up".
Top diplomats from the seven major advanced economies were in Italy for their annual two-day meeting which had initially been expected to focus on talks with new US Secretary of State Rex Tillerson about hotspots like Libya, Iran and Ukraine.
But the agenda is now likely to be dominated by last week's suspected chemical weapons attack on a rebel-held Syrian town that killed at least 87 civilians, and the US cruise missiles fired at a Syrian air base in retaliation.
It was the first time Washington has intervened directly against the regime of Assad, who is fighting a civil war with the backing of Russia and Iran, and the G7 ministers will deliberate the West's next steps.
WATCH: US to 'confront crimes against humanity'
Several rounds of UN-backed peace talks have failed to end the conflict, which has killed more than 320,000 people since March 2011.
The G7 gathering in the Tuscan city of Lucca brought foreign ministers from the United States as well as Britain, Canada, France, Germany, Italy and Japan to the 15th century Ducal Palace.
The ministers, ushered out of the Tuscan sunshine and into the fresco-decorated palace for their first working session, were set to go on a walking tour of the city's historic centre later before a working dinner in the majestic Palazzo Orsetti.
'End Assad support'
Washington's retaliation was slammed by Iran and North Korea and put it on a direct diplomatic collision course with Moscow, where Tillerson heads on Tuesday for talks with Russian counterpart Sergei Lavrov.
The US urged Russia on Sunday to rein in the Syrian regime, warning that any further chemical attacks would be "very damaging" to their relationship and suggesting any peace deal would be difficult with Assad in power.
"We need to make it clear to Putin that the time to back Assad has gone," Johnson said Monday, warning that Putin was "damaging Russia" by supporting Assad.
He called on Moscow to do "everything possible to bring about a political settlement in Syria and work with the rest of the international community to ensure that the shocking events of the last week are never repeated".
Tillerson would "deliver that clear and coordinated message to the Russians", he said.
French Foreign Minister Jean-Marc Ayrault said the pressing task for the G7 was to "find a political solution, a political transition" in Syria, particularly if the West wanted to triumph over the so-called Islamic State (IS).
"The fight against terrorism cannot be effective if we do not link it to resolving the Syrian situation," he said.
WATCH: No peace in Syria under Assad, US says
'Crime against innocents'
Italy has arranged a last-minute meeting on Tuesday between the G7 ministers and their counterparts from Jordan, Qatar, Saudi Arabia, Turkey and the United Arab Emirates.
Italian media said the aim was "to avert a dangerous military escalation".
Japan's Foreign Minister Fumio Kishida said he told Tillerson that Tokyo supports the US in its push to "deter the spread and use of chemical weapons", and discussed the pressing North Korean nuclear threat.
Japan hopes the strong US response on Syria will also put pressure on the isolated country, which is showing signs of preparing for its sixth nuclear test and more test-firings of ballistic missiles.
"We agreed that the role of China is extremely important. Japan and the United States will jointly call on China to play a bigger role," Kishida told reporters after meeting Tillerson in Lucca.
Tillerson spent the morning at a WWII ceremony at the site of a Nazi massacre in Sant'Anna di Stazzema near Lucca.
"We rededicate ourselves to holding to account any and all who commit crimes against the innocents anywhere in the world," he said after he and other ministers lay a red wreath at the foot of the site's memorial.
WATCH: US attack crosses red lines, Assad allies say