Western powers are weighing new sanctions on Russia and Syria for a deadly chemical attack last week, as leaders seek to present a united front ahead of Secretary of State Rex Tillerson's visit to Moscow.

"The game has now been changed and it's important that message should be heard from the Americans to the Russians," British Foreign Secretary Boris Johnson told reporters on the first of two days of talks between foreign ministers of the Group of Seven (G7) nations in the ancient Tuscan town of Lucca.

"It's very important in these circumstances for the world to present a united front and for there to be absolutely no ambiguity about the message," Mr Johnson said, when asked why he chose to cancel talks due in Moscow and attend the G7 in Italy instead.

The US strike, in response to the April 4 attack that killed dozens of people, including many children, was Washington's first intervention against the regime of President Bashar al-Assad and pushed the country's six-year civil war to the top of the agenda for the G-7 meeting. Ministers were also expected to discuss countering terrorism, grappling with North Korea's nuclear program and the European refugee crisis.