US demands that Russia, Iran halt Assad's chemical attacks in Syria

US has been under fire not on ending Assad's civil war against his domestic opposition

The United States warned that Syrian leader Bashar al-Assad must be held accountable for chemical weapons attacks and demanded that Russia and Iran bring their ally to heel.

"While we continue to monitor the terrible situation, it is clear that this is how Bashar al-Assad operates, with brutal, unabashed barbarism," US Secretary of State Rex Tillerson said.

President Donald Trump and Tillerson issued separate written statements after a suspected chemical attack left dozens of Syrians dead in a rebel-held town outside.

The US administration has been under fire for concentrating its efforts on the defeat of the jihadist Islamic State group and not on ending Assad's civil war against his domestic opposition.

But Tillerson, who will visit Moscow next week, said the latest attack underlined the need for Russia and Iran to save the civil war peace process by reigning in their ally's excesses.

"Those who defend and support him, including Russia and Iran, should have no illusions about Assad or his intentions," he said.

"Anyone who uses chemical weapons to attack his own people shows a fundamental disregard for human decency and must be held accountable.

"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," Tillerson said.

"As the self-proclaimed guarantors to the ceasefire negotiated in Astana, Russia and Iran also bear great moral responsibility for these deaths.