Secretary of State Rex Tillerson blamed Russia and Iran for enabling Syrian President Bashar Assad to carry out a chemical weapons attack on rebel-held territory.

"While we continue to monitor the terrible situation, it is clear that this is how Bashar al-Assad operates: with brutal, unabashed barbarism," Tillerson said Tuesday. "We call upon Russia and Iran, yet again, to exercise their influence over the Syrian regime and to guarantee that this sort of horrific attack never happens again. As the self-proclaimed guarantors to the ceasefire negotiated in Astana, Russia and Iran also bear great moral responsibility for these deaths."

That's the strongest condemnation of the latest chemical weapons attack by a member of President Trump's administration, which was initially back-footed by news of the gas attack in Syria. Lawmakers in both parties accused Tillerson of emboldening Assad by suggesting he might remain in power after the civil war's conclusion and British Prime Minister Theresa May pointedly called for Assad's departure.

"Anyone who uses chemical weapons to attack his own people shows a fundamental disregard for human decency and must be held accountable," Tillerson said. "It is also clear that this horrific conflict, now in its seventh year, demands a genuine ceasefire and the supporters of the armed combatants in the region need to ensure compliance."

With that, he outstripped even May's statement, which technically reserved judgement about whether Assad was responsible for the latest chemical weapons attack. "If proven, this will be further evidence of the barbarism of the Syrian regime," May said. "I'm very clear that there can be no future for Assad in a stable Syria which is representative of all the Syrian people and I call on all the third parties involved to ensure that we have a transition away from Assad."