The vocal dispute between Russia and the United States over Syria complicates what has been a feature event; U.S. political drama for months has been about Russian meddling in USA elections and the blowback from Trump.
The US blames Syrian President Bashar al-Assad for the carnage, but the Syrian military denies responsibility for the attack.
Russian Federation says it will host the foreign ministers of Iran and Syria for a three-way meeting.
Mattis also sought to downplay concerns about Pentagon mission creep, saying the US military's focus in Syria was still fighting Islamic State militants, not becoming entangled in Syria's messy civil war between Assad and his opponents.
An anonymous senior US official said April 10 that the Pentagon now believes that Russian Federation had advance knowledge of the recent chemical weapons attack in Syria that left nearly 70 dead, AP reported. Tillerson had strong words for Russian Federation after the G7 meeting.
The secretary of state commenced his visit to Italy by traveling up a winding mountain road to Sant'Anna di Stazzema, the Tuscan village where the Nazis massacred more than 500 civilians during World War II.
"The United States should revoke authorisations and re-impose sanctions on Iranian airlines found guilty of such support, and should bar U.S. companies from selling aircraft to Iran until the Iranian regime ceases using commercial airliners for illicit military purposes", the letter said.
Russian Federation has claimed Tuesday's deaths in Khan Sheikhoun were caused by a Syrian regime airstrike on a rebel-controlled chemical weapons facility.
Tillerson said on Thursday that Russian Federation had "failed in its responsibility" to remove Syria's chemical weapons under a 2013 agreement, which he argued showed Russian Federation was either complicit with the gas attacks or "simply incompetent". "First and foremost, we must defeat ISIS", he said, when asked about the possibility of deposing Assad. Tillerson, who had no experience of worldwide diplomacy before being appointed Secretary of State, is facing a situation that would test the most seasoned of diplomats, heading to Moscow with only verbal messages of support from key allies.
It also is unclear if Trump, who has expressed skepticism of multilateral institutions such as the European Union and United Nations, will have patience for the protracted negotiations that a comprehensive deal on Syria would require.
The ties between Trump staffers, past and present, to Russia, was aided by Trump's willingness to start fresh with Russian President Vladimir Putin, whom he defended in the U.S. media.
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Mattis said there was no doubt that Damascus was responsible.
President Trump recently said, "That attack on children yesterday had a big impact on me".
Since the United States launched airstrikes against Assad's forces in retaliation for a chemical attack on civilians last week, Trump administration officials have offered mixed messages about whether Washington believes Assad definitely must surrender power, and when.
Russia's foreign ministry said it hoped for "productive talks" with Tillerson to understand "to what extent the United States understands the need for stabilising and normalising bilateral relations".
Tillerson and Lavrov discussed the missile strike in a phone call Saturday. As of this weekend, the talks were still on.
Yet the USA has no proof of Moscow's involvement, said the official, who wasn't authorized to speak publicly on intelligence matters and demanded anonymity. Russell Caldwell Sunday to thank them and their crews for their "professionalism and quick response" when conducting the air missile strike, according to a Navy news release.
Officials, in a declassified four-page report on the strike and chemical attack, said Tuesday they were "confident" that the Assad regime used sarin gas against their own people.
"The strikes aren't necessarily a bad thing for Russian Federation", said Andrew Tabler, a fellow with the Washington Institute for Near East Policy.
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In her CNN interview Sunday, Haley suggested that a change of government in the war-ravaged nation was inevitable, and said removing Assad was now a United States priority.