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 are 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.
The gathering in the Tuscan city of Lucca, which begins at 1430 GMT, groups foreign ministers from the United States and Britain, Canada, France, Germany, Italy and Japan.
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 his Russian counterpart Sergei Lavrov.
The US stepped up the pressure on Sunday on Russia 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.
Tillerson enraged Moscow by asking if it was possible Russia did not know about Syria's chemical arms, and called on the country to fulfil the obligation it made to the international community to guarantee the elimination of the weapons.
"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 had cancelled a scheduled visit to Moscow "to continue contact with the US and others" ahead of Tillerson's Russian trip.
He called on Russia 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.