Wednesday’s suspected chemical attack claimed 89 lives
The search for a unified approach to the Syria conflict after last week’s suspected chemical attack looks set to dominate talks between the G7 group of leading nations in Italy on Monday.
Foreign ministers will focus on how to pressure Russia to distance itself from Syrian President Bashar al-Assad.
Allies will also be seeking clarity from the US on its Syria policy, after some apparently mixed messages.
US Secretary of State Rex Tillerson strongly criticised Russia on Sunday.
He said it had failed to prevent Syria from carrying out a chemical attack on the rebel-held town of Khan Sheikhoun last Wednesday which left 89 people dead.
But he also clarified there had been "no change to our military posture" in Syria following a retaliatory US strike against a Syrian airbase, and that Washington’s "first priority" in Syria was to defeat the Islamic State terror group.
Those comments came a day after the US ambassador to the United Nations said there was no way to stabilise Syria with Mr Assad as president.
"In no way do we see peace in that area with Assad as the head of the Syrian government," Nikki Haley told NBC’s "Meet the Press".
However, she had said last week that Mr Assad’s removal was no longer a US priority.
Analysis: BBC diplomatic correspondent James Robbins in Lucca, Italy
The next two days will be dominated by a collective search for arguments to persuade President Vladimir Putin he must now end Russia’s military support for President Assad and help accelerate a negotiated political transition.
UK Foreign Secretary Boris Johnson is expected to press the case for new sanctions against Russia if they don’t give ground. Mr Tillerson wants to go on from here to Moscow able to confront the Russians with a strong set of demands backed by key US allies.
But Mr Tillerson made clear over the weekend that Washington still regards the number one priority in Syria to be the elimination of so-called Islamic State.
Meanwhile, Russia and Iran, President Assad’s key military backers, are threatening retaliation if there are any further American air strikes and the task of breaking the present deadlocks over Syria remains enormous.
The G7 consists of Canada, France, Germany, Italy, Japan, the UK, and the US, with the European Union also represented. Ministers are meeting in the city of Lucca, in Tuscany on Monday and Tuesday.
On Tuesday, Mr Tillerson will continue from the G7 to Moscow, where he will meet his Russian counterpart, Sergei Lavrov.
Russia is the Syrian government’s main ally, and helped facilitate a 2013 agreement to destroy Syria’s chemical arsenal.
Mr Tillerson said that Russia’s failure to ensure Syria’s chemical weapons stockpiles were destroyed enabled the attack.
Syria has denied using any chemical agents, and Russia says the US gave no evidence Syria has chemical weapons.
A joint command centre comprising the forces of President Assad’s allies, including Russia and Iran, issued a statement saying the US strike had crossed "red lines".
"From now on we will respond with force to any aggressor or any breach of red lines from whoever it is and America knows our ability to respond well," said the statement published by the group on media outlet Ilam al Harbi (War Media).