(Adds Blinken quotes, background on nuclear threats)
WASHINGTON, Sept 30 (Reuters) - The United States has not yet seen Russia take any action that suggests it is contemplating the use of nuclear weapons amid its invasion of Ukraine, U.S. Secretary of State Antony Blinken said on Friday, despite what he called "loose talk" by Russian President Vladimir Putin about their possible use.
"We are looking very carefully to see if Russia is actually doing anything that suggests that they are contemplating the use of nuclear weapons. To date, we've not seen them take these actions," Blinken told a news conference in Washington with his Canadian counterpart.
"This kind of loose talk about nuclear weapons is the height of irresponsibility and it's something that we take very seriously," Blinken said.
Putin on Friday proclaimed Russia's annexation of a swathe of Ukraine, the biggest annexation in Europe since World War Two. Putin also vowed to press ahead with what he calls a "special military operation" in Ukraine. Russia launched its invasion in February.
In recent weeks, Putin explicitly warned the West that Russia would use all available means to defend Russian territory and accused the West, without offering evidence, of discussing a potential nuclear attack on Russia. Putin on Friday said the United States had set a precedent when it had dropped two atomic bombs on Japan in 1945, but stopped short of issuing new nuclear warnings against Ukraine.
Pentagon spokesperson Brigadier General Patrick Ryder this week said the United States had not seen any changes that would lead it to alter the posture of American nuclear forces.
Putin controls the world's largest nuclear arsenal, including a new generation of hypersonic weapons and 10 times more tactical nuclear weapons than the West. (Reporting by Simon Lewis and Humeyra Pamuk; Editing by Chizu Nomiyama and Will Dunham)