US Secretary of State Mike Pompeo on Wednesday called Russia's delivery of the S-300 surface-to-air missile system to Syria a "serious escalation".
“I’m certainly not going to comment on our intention on how we will address that, but my comments before were true. Having the Russians deliver the S-300 into Syria presents greater risk to all of those in the affected areas and to stability in the Middle East. We consider this a very serious escalation,” Pompeo said in remarks to the media.