Russia is expanding and modernizing its nuclear arsenal, U.S. Defense Secretary Lloyd Austin said on Friday, at a time Russian President Vladimir Putin has repeatedly suggested he could use nuclear weapons to protect Russia.

“Russia is also modernizing and expanding its nuclear arsenal,” Austin said at a ceremony for the incoming commander of U.S. Strategic Command, which oversees the United States’ nuclear arsenal.

“And as the Kremlin continues its cruel and unprovoked war of choice against Ukraine, the whole world has seen Putin engage in deeply irresponsible nuclear saber-rattling,” Austin added.