U.S. Secretary of State Mike Pompeo warned Russia on Wednesday regarding its delivery of the S-300 missile-defense system to the Syrian regime, calling it a “very serious escalation” in regional security.

Without specifying how the United States would respond to the transfer, Pompeo said “having the Russians deliver the S-300 into Syria presents greater risk to those in the affected areas and to stability in the Middle East.We consider this a very serious escalation.”

Russia confirmed the transfer late last week.

“The deliveries started already,” said Russian Foreign Minister Sergey Lavrov. “The measures we will take will be devoted to ensure 100 percent safety and security of our men in Syria, and we will do this.”

The delivery was in response to an Israeli airstrike earlier this month that was mistakenly shot down by a Russian military plane, killing all 15 of its crew members.