U.S. warns Putin of 'catastrophic' consequences if nuclear weapons used in Ukraine

The United States would respond decisively to any Russian use of nuclear weapons against Ukraine and has spelled out to Moscow the "catastrophic consequences" it would face, U.S. National Security Adviser Jake Sullivan said on Sunday.

Sullivan's remarks represented the latest American warning following the thinly veiled nuclear threat made by Vladimir Putin last Wednesday in a speech in which the Russian president also announced his country's first wartime military mobilization since World War Two.

"If Russia crosses this line, there will be catastrophic consequences for Russia. The United States...