Bashar al-Assad's regime is chiefly responsible for a suspected chemical attack that has killed scores of civilians in Syria, EU president Donald Tusk said Wednesday.

"The Syrian regime (has) the primary responsibility for the atrocities," Tusk said, adding: "but all those who support the regime share the moral and political responsibility."

At least 72 civilians, among them 20 children, were killed in Tuesday's attack in rebel-held Khan Sheikhun, and dozens more were left gasping for air, convulsing, and foaming at the mouth, doctors said.

"The indiscriminate killing of children, women and men with chemical weapons is another tragic reminder of the brutality of this conflict, and the Syrian regime," Tusk said during a visit to Greece.