The United States has "no doubt" that Bashar al Assad is responsible for a chemical weapons attack in Syria that killed dozens of civilians, including children, the defence secretary has said.
General James Mattis defended US airstrikes that targeted a Syrian airbase in retaliation for the 4 April attack on a rebel-held area in Idlib province.
He told reporters that Washington's military strategy in Syria had not changed even after the strikes, saying "our priority remains the defeat" of the Islamic State group.
The attack in Khan Sheikhoun killed at least 80 people. Witnesses described seeing victims of the attack choking, fainting and foaming at the mouth.
Mr Assad and his main international backer, Russia's president Vladimir Putin, deny that Damascus was behind the massacre.
General Mattis, briefing reporters at the Pentagon, said: "I have personally reviewed the intelligence and there is no doubt the Syrian regime is responsible for the decision to attack and for the attack itself."
He said Mr Assad had used chemical weapons several times in the past, and urged Damascus to "think long and hard" before making a reckless attack again.
The attack provoked outrage across the globe, rekindling diplomatic activity to try and isolate Mr Assad and push Russia to abandon him.
US secretary of state Rex Tillerson arrived in Moscow on Tuesday for talks with his Russian counterpart Sergei Lavrov.
Mr Tillerson, who as former CEO of Exxon Mobil had close tied to Russia, is the first Trump cabinet member to visit Moscow.
He said before departing that "the reign of the Assad family is coming to an end" and issued an ultimatum to Russia: either side with world powers in seeking an end to Syria's six-year war, or continue the alliance with Iran and the militant group Hezbollah in supporting the Assad regime.
Earlier in the day, G7 foreign ministers rejected British Foreign Secretary Boris Johnson's calls to broaden sanctions against Russia and Syria.
As Mr Tillerson began his visit, Moscow and Washington traded barbs.
Mr Putin has said Washington's accusations against the Syrian government over the chemical attack resemble the claims made before the US invaded Iraq in 2003.
The US accused Moscow of trying to cover up on Damascus' behalf, and of seeking to "confuse the world" with a campaign of misinformation.