"That's a butcher", Trump said.
Trump said that Secretary of State Rex Tillerson had completed a successful meeting with Russian President Vladimir Putin in Moscow, where "things went pretty well". "We want Russian Federation to use its influence over the Assad regime to stop the madness and the cruelty we see every day on the ground".
While the U.S. says not all available intelligence has been released to protect its sources and the methods, it affirms that sarin gas was released from a pipe dropped from the air.
Foreign Secretary Boris Johnson has come under fire over his handling of the G7 gathering, which put off decisions on any further sanctions until the outcome of an investigation into the chemical weapon attack.
Moscow has stood by Syrian regime leader Bashar al Assad, saying the poison gas belonged to rebels, an explanation Washington dismisses as beyond credible.
But the day before the United Nations vote, Russian President Vladimir Putin called for a United Nations investigation into the incident -- though he blamed the incident on rebels, not Assad.
US intelligence officials have intercepted communications that feature Syrian soldiers preparing for last week's deadly chemical attack, according to a report.
The US report alleges that the chemical agent was delivered by a Syrian Su-22 fixed-wing aircraft that flew over the village of Khan Sheikhun at the time of the attack, which killed at least 87 civilians, including 31 children, on 4 April. "After today's vote to hold Syria accountable it's: A strong day for the U.S., a weak day for Russian Federation, a new day for China & doomsday for Assad", the United States ambassador to the UN, Nikki Haley, said on Twitter.
"We have not seen any facts, not even hints of facts". He said it was "unacceptable that the OPCW analyses the evidence not on location".
The U.S. struck Syria with 59 missiles launched from two Navy ships in the Mediterranean last week hitting the Shayrat base, near the city of Homs in Syria. This has been the case since the presidency of Obama and the USA does not need to wait for Assad to use chemical weapons to launch strikes. Western leaders backed the US action, saying Assad had brought it on himself.
Putin, in a television interview ahead of the talks, was more blunt about the new chill in ties, and firmly rejected Washington's view of the Syrian attack.
Lavrov told Tillerson Moscow was hoping to understand Washington's "real intentions" and warned that the Kremlin considered it "fundamentally important" to prevent more "unlawful" United States strikes against its ally Syria.