The United States is warning Russia that it will face catastrophic consequences if it uses nuclear weapons in Ukraine. U.S. national security adviser Jake Sullivan appeared on ABC on Sunday.

Jake Sullivan: “We have communicated directly, privately to the Russians, at very high levels, that there will be catastrophic consequences for Russia if they use nuclear weapons in Ukraine. We have been clear with them and emphatic with them that the United States will respond decisively, alongside our allies and partners, and we have protected those communications, which we have done privately to the Russians, but they well understand what they would face if they went down that dark road.”

Jake Sullivan’s comment comes days after Russian President Vladimir Putin threatened to use nuclear weapons to protect Russia. This comes as Russia is moving toward annexing parts of occupied Ukraine, and vowing to protect the areas as if they were Russian territory. Referendums on joining Russia are being held in four areas.