US Secretary of State Rex Tillerson warned Russia on Wednesday to re-think its support for Bashar al-Assad's Syrian regime, after it was blamed for a major chemical attack.

"There is no doubt in our mind that the Syrian regime under the leadership of Bashar al-Assad is responsible for this horrific attack," Tillerson told reporters at the State Department.

"And we think it is time for the Russians to really think carefully about their continuing support for the Assad regime," he said.