MOSCOW (AP) — U.S. Secretary of State Rex Tillerson arrived in Moscow Tuesday with an ultimatum for Russia: Side with the U.S. and likeminded countries on Syria, or embrace Iran, militant group Hezbollah and embattled Syrian leader Bashar Assad.

It wasn’t clear what the punishment would be for a Russian government that has used its military might to help Assad and his array of allies score a series of battlefield successes in their six-year war with Syrian opposition groups.

Russian President Vladimir Putin immediately showed that he wouldn’t back down, saying Russia knew about planned “provocations” to blame Syria’s government for using chemical weapons. He said the U.N. should first investigate the attack.

“It reminds me of the events in 2003 when U.S. envoys to the Security Council were demonstrating what they said were chemical weapons found in Iraq,” Putin told reporters on Tuesday. “We have seen it all already.”

Tillerson is in Moscow to meet with Russian officials about the Syria civil war. It is the first official trip to Russia by a member of President Donald Trump’s Cabinet. It’s unclear if Putin and Tillerson will meet.