Putin swipes at US in hostile speech amid annexation of Ukraine, says it set 'precedent' for nuclear warfare

Russian President Vladimir Putin came out swinging at the U.S. on Friday in an aggressive speech as he announced the annexation of four Ukrainian regions. 

In a message that was already expected to be hostile, Putin further escalated threats against Ukraine and the West by saying the U.S. set a "precedent" of nuclear warfare by hitting Japan with atomic bombs during World War II.

Fears have mounted since last week when Putin said he was "not bluffing" after he warned that Moscow would use "all weapons systems available" to defend Russian territory.