U.S. intelligence experts assess Syrian President Bashar al-Assad's military is probably hoarding the weapons in munitions depots at Shayrat airfield near Homs in central Syria, the United States military's Central Command spokesman Colonel John Thomas said. The next step would be to use United Nations talks to negotiate a political transition that could include Assad leaving power.
On Monday Tillerson raised fresh expectations for aggressive US action - and not only in Syria - as he visited the site of a World War II Nazi massacre in central Italy, saying the USA would hold to account "all who commit crimes against the innocents anywhere in the world".
A joint command centre made up of the forces of Russia, Iran and Lebanon's Hezbollah militia said on Sunday that the US strike crossed "red lines" - the term Washington has used in the past to refer to Syria's use of poison gas.
"It is clear to all of us that the reign of the Assad family is coming to an end".
"Getting Assad out is not the only priority". Whether that affects their already laid-out plans is yet to be seen, but they now have a better understanding of the new world game of Risk board that has been created. Thirdly, get the Iranian influence out.
Just a little more than a week ago, top administration officials were signaling that removing Assad is no longer a US priority.
"If you look at his actions, if you look at the situation, it's going to be hard to see a government that's peaceful and stable with Assad", she said.
Retaliating for barrel bombs would require a major shift in USA policy since rebels say the weapons are used nearly daily.
But Tillerson was more equivocal, saying that the priority was the defeat of ISIS. It is the first meeting of USA allies since President Donald Trump ordered the bombardment on the Shayrat airbase in western Syria last week.
Assad's regime is accused of regularly using barrel bombs against civilians, making Spicer's comments potentially a bold new line in the sand for the Trump administration.
And yet Mattis kept the door open to further USA strikes against the Syrian regime if it used nerve agents again against the its people.More news: Ghost in the Shell
The tensions between Russian Federation and United States are unlikely to escalate any further, US Secretary of Defense James Mattis told reporters at a briefing in the Pentagon late Tuesday.
"But in terms of stopping, the chemical weapons attacks against the people of Syria, I hope it succeeds", he said.
"The official said the presence of the drone couldn't have been a coincidence, and that Russian Federation must have known the chemical weapons attack was coming and that victims were seeking treatment", the AP writes.
Syria has given a murky account of what happened, but denies deliberately carrying out a chemical attack and blames "terrorist groups" for the deaths.
Officials said a wealth of evidence supported their claim, citing open-source videos, satellite imagery, intercepted communications, and physiological evidence from the attack site. But in interviews broadcast Sunday, Tillerson said he sees no reason for retaliation from Moscow because Russian Federation wasn't targeted.
Haley on Friday warned that the United States was prepared to take further actions in Syria, but the Trump administration has refused to be drawn on what those next steps would be.
Even since the missile strikes, signals have been mixed. G7 ministers, in the face of conflicting signals on United States policy towards Syria and Russian Federation, appear to have chosen to postpone any big decision until after those talks.
"It's very hard to understand how a political solution could result from the continuation of the Assad regime", McMaster said on "Fox News Sunday".
The official said declassified US intelligence disputes Kremlin narratives, including one describing the April 4 poison gas attack as the result of an explosion after a strike on a Syrian munitions depot where terrorists were holding chemicals.
"I hope Russian Federation is thinking carefully about its continued alliance with Bashar al-Assad, because every time one of these horrific attacks occurs, it draws Russian Federation closer into some level of responsibility", Tillerson said on ABC's "This Week with George Stephanopoulos" on Sunday.
White House spokesman Sean Spicer said Monday that the use of barrel bombs, which government forces have repeatedly employed throughout the six-year-old conflict, might bring a US response. Russian Federation flatly denies the allegations.