According to U.S. National Security Advisor Jake Sullivan, the state would react 'decisively' if Russia used nuclear weapons against Ukraine. U.S. has also explained to Moscow the "catastrophic consequences" that would follow.

Following Vladimir Putin's veiled nuclear threat last Wednesday in a speech in which he also declared his nation's first wartime military mobilization since World War Two, Sullivan's statements represented the most recent American warning.

In his comments on Sunday, Sullivan said the United States had privately "spelled out in greater detail exactly what that would mean" to Moscow but did not specify the nature of the intended reaction. According to Sullivan, the United States and Russia have been in regular, direct contact recently to discuss the situation in Ukraine and Putin's actions and threats.

"If Russia crosses this line, there will be catastrophic consequences for Russia. The United States will respond decisively," Sullivan told NBC's "Meet the Press" program.

Source: The Guardian