Turkish Foreign Minister Mevlut Cavusoglu earlier in the day said the regime still had "chemical weapons capacity" as top diplomats gathered for G7 talks in Italy.
U.S. Secretary of State Rex Tillerson says Russian Federation must choose between aligning itself with the U.S. and likeminded countries or embracing Syrian President Bashar Assad, Iran and the militant group Hezbollah.
Before setting off for Moscow, Tillerson told a G7 foreign ministers' meeting in Italy that Russian Federation should rethink its alliance in the light of the latest alleged chemical attack in Syria. "I welcome this renewed harmony". In Italy, diplomats spoke of possible new sanctions on Syria's and Russia's militaries, and additional US military action if Assad's forces continue attacking civilians.
The talks got off to a frosty start, with the Russian politician branding the American cruise missile strike "illegal" and criticising "confusing and sometimes openly contradictory ideas" from the Trump administration on the US-Russia relationship and global issues.
"The United States will not passively stand by while Assad murders innocent people with chemical weapons, which are prohibited by worldwide law and which were declared destroyed", Pentagon chief James Mattis said Monday.
The Syrian government has strongly denied responsibility for the attack in Idlib, which left more than 80 people dead.
And they say Russian Federation faces a choice: Align with the US and its coalition fighting the Islamic State group in Syria, or stick with Assad and his other ally, Iran.
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Tillerson's Moscow talks hinge on new United States leverage over Syria.
"The United Kingdom therefore shares the U.S. assessment that it is highly likely that the regime was responsible for a sarin attack on Khan Sheikhoun on April 4", Mr Rycroft said. He said the United Nations should first investigate the attack.
Tillerson says the United State is confident in its assessment that Syria used chemical weapons in a bombing last week and alleged that Syria has used such weapons more than 50 times in the past.
Following on from the Tillerson visit, Moscow has announced Lavrov will host three-way talks with his counterparts from Syria and Iran later this week.
Another U.S. official cautioned that no final American determination has been made that Russian Federation knew ahead of time that chemical weapons would be used.
Mr Johnson backed U.S. calls to find a political solution and said the G7 leaders were ready to work with Russian Federation to end the violence in Syria.