Leading US allies have failed to reach agreement on new sanctions on Syria and Russian Federation in the wake of a chemical attack that killed more than 80 people.
Ahead of Tillerson's trip, Washington has stepped up the pressure on Moscow to rein in its Syrian ally President Bashar al-Assad, warning Damascus that further use of chemical weapons could bring fresh U.S. military retaliation.
Worldwide reaction has been intense since the USA fired 59 Tomahawk missiles at the Shayrat Airfield in Syria Friday, which housed the warplanes the United States believes were used in last week's chemical weapons attack on civilians.
Tillerson said that Russian Federation had failed to uphold commitments it made in 2013 to guarantee the Syrian regime got rid of chemical weapons.
Mattis's warning came as White House spokesman Sean Spicer appeared to lower the threshold for new USA action against Assad to include barrel bombs, a crude yet hugely destructive weapon of choice for the Syrian leader.
"If you gas a baby, if you put a barrel bombing to innocent people, I think you can see a response from this president".
But US officials later appeared to go back on Spicer's remarks. "(Spicer) did not signal a change in administration policy", an official said.
"The raging civil war in Syria, the violent conflicts around the world", he said, "require an worldwide community that pulls together, that holds each other to account and that strives everyday not to repeat the awful mistakes of the past".
Trump discussed Syria during separate telephone calls Monday with British Prime Minister Theresa May and German Chancellor Angela Merkel.
G7 foreign ministers warned against military escalation in Syria Tuesday, throwing their support behind US Secretary of State Rex Tillerson as he readies for crunch talks in Moscow, a key Damascus ally. US Secretary of State Rex Tillerson arrives the meeting of Foreign Ministers of the G7 countries in Lucca, Italy, Monday, April 10, 2017.
Foreign ministers from the Group of Seven (G7) major industrialised nations met in Italy on Monday, looking to put pressure on Russian Federation to break its ties with Syrian President Bashar al-Assad.
The annual two-day meeting had initially been expected to focus on talks with Tillerson about hotspots like Libya, Iran and Ukraine.
USA ambassador to the UN Nikki Haley suggested regime change in Syria was inevitable in the wake of last week's chemical weapons attack that was widely blamed on the Assad regime.
Several rounds of UN-backed peace talks have failed to end the Syrian conflict, which has killed more than 320,000 people since March 2011.
A meeting of the "likeminded" countries was hastily arranged on the sidelines of a Group of 7 industrialized economies in Italy, days after the USA for the first time launched airstrikes against Syrian President Bashar Assad's forces.
And Germany's Gabriel stressed that "without Moscow and Tehran there will be no solution in Syria".
"There's an opportunity to bring renewed energy to the political process".
"There is no consensus on additional new sanctions", Italian Foreign Minister Angelino Alfano said.
"It is clear to all of us that the reign of the Assad family is coming to an end", he said.
Japan is also hoping that the strong USA response on Syria will also put pressure on Pyongyang, 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".
Yet the US has sent mixed messages about what it wants in the short term; it's not even clear whether Washington intends to help oust Assad.
In a letter to Trump, two Republican lawmakers Senator Marco Rubio and Congressman Peter Roskam urged him to suspend aircraft sales to Iran arguing that Tehran is using commercial passenger aircraft to supply its worldwide network of terror proxies, including the Assad regime in Syria.