Rex Tillerson issues an ultimatum to Russia ahead of talks

US Secretary of State Rex Tillerson landed in Moscow on Tuesday with a message for Russia: Side with the US and like-minded countries on Syria, or join Iran, Hezbollah and Bashar Assad.

It was less clear what the consequences would be for a government that helped Assad and his allies succeed in their six-year war with Syrian opposition groups.

Russian President Vladimir Putin immediately refused to back down, saying Russia knew about organised "provocations" to blame Syria's government for using chemical weapons, and urged the UN to investigate the attack.

Putin told reporters: "It reminds me of the events in 2003 when US envoys to the Security Council were demonstrating what they said were chemical weapons found in Iraq. We have seen it all already."

Tillerson is in Moscow to speak with Russian officials about the Syrian war, in the first official trip to Russia by a member of President Trump's cabinet.

It is unclear whether Tillerson will meet Putin.

Before he left for Russia, Tillerson told reporters that Moscow either did not take its obligation to rid Syria of chemical weapons seriously, or it had been incompetent.

He also said it was clear that the US did not see a role for Assad in Syria's future, as he had lost legitimacy.

"It is clear to all of us that the reign of the Assad family is coming to an end," he said. "But the question of how that ends and the transition itself could be very important in our view to the durability, the stability inside of a unified Syria."

"That's why we are not presupposing how that occurs," Mr Tillerson added.

Russia's foreign ministry said that it hoped for "productive talks", and that the outcome of discussions was important "for the overall atmosphere on the world stage".