Days later, after a chemical weapons attack on Syrian civilians that the US blames on Assad's regime, the Trump administration sharpened its rhetoric and launched several dozen cruise missiles at a Syrian airfield, marking the first USA military effort against the Assad government.
Instead, Tillerson emphasized the administration's priority in defeating Islamic State forces in Syria and said little about the U.S.'s preference on Assad's future.
Russian Federation had agreed to ensure Syria's chemical weapons stockpiles were destroyed - and its failure to do this enabled the attack, he said.
The US ambassador to the UN, Nikki Haley, told CNN: "Either they knew that there were chemical weapons and they knew there was going to be chemical weapon use, and they just hid it from the worldwide community, or they are being played for fools by Assad".
Tillerson is arriving at a fragile point in U.S. -Russia relations, where he will have to confront the Kremlin's anger over the missile strikes in Syria along with suspicion at home that Moscow may have been complicit in the Syrian government's chemical weapons attack.
However, he stopped short of accusing Russian Federation of being directly involved in the planning or execution of the attack, saying he had not seen "any hard evidence" to suggest the country was an accomplice to Mr Assad.
A total of 87 people, including many children, died in the suspected sarin attack on the rebel-held town of Khan Sheikhoun in Idlib province on Tuesday.
Later this week US Secretary of State Rex Tillerson will travel to Moscow, while Foreign Secretary Boris Johnson will garner support at a G7 summit of foreign ministers.
The Russian leader also said the USA attack was a "show" and a "preplanned action".
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Rex Tillerson says he's "seen no evidence" that Russian Federation is an ally of Syrian President Bashar al-Assad.
The US responded to the poison gas attack, which is thought to have killed about 70 civilians, with a cruise missile strike on a Syrian regime air base on Friday.
"What we've seen is, you know, in our meetings this week, we were told of the evidence", she said.
"Russia could be part of the solution", he said, adding that now, much of the world thinks Russian Federation is "part of the problem".
Tillerson said on CBS' Face the Nation that Russians were not targeted by the strikes.
He and Senator Lindsey Graham said Trump deserves the respect of the American people because he took action, unlike Obama and his administration.
Hassan Rouhani and Vladimir Putin, the Iranian and Russian presidents, said on Sunday that USA airstrike against Syria was an act of invasion which ran contrary to worldwide law and the United Nations charter.
The US Navy's Third Fleet forward-deployed strike group, headed by the supercarrier USS Carl Vinson and supported by guided-missile destroyers and cruisers, was scheduled to visit Australia but it was announced on Saturday they had received orders to re-route toward the Korean Peninsula. "Secondly, we don't see a peaceful Syria with Assad in there".
Mr Johnson's visit to Moscow was cancelled following a late night phone call with Mr Tillerson, prompting fierce criticism of the Foreign Secretary.