US President Donald Trump launched a fierce attack on Russia.

"Putin is backing a person that's truly an evil person. And I think it's very bad for Russia, I think it's very bad for mankind, it's very bad for this world," Trump said in an interview broadcast on Wednesday.

"But when you drop gas, or bombs, or barrel bombs, they have these massive barrels with dynamite and they drop them right in the middle of a group of people, and in all fairness, you see the same kids - no arms, no legs, no face. This is an animal," Trump concluded.

US Secretary of State Rex Tillerson said yesterday that Assad's rule in Syria is nearing an end, warning Russia that it may become "irrelevant" if it continues to support Assad's regime.

Tilerson spoke to reporters shortly before he flew to Moscow to meet Russian President Vladimir Putin. "As far as the future of Bashar al-Assad is concerned, it is important for us to embark on a political process that will lead to a final conclusion on how the Syrian regime will be governed," Tilerson said.

"Our policy calls for a united Syria under the rule of the people of Syria, and I think it is clear to us that the Assad regime is coming to an end, but the question of how it will end and how the transition itself will be carried out can be very important in terms of internal stability of a United Syria," Tillerson added.