The Russian Foreign Ministry has protested to the US after a veteran senator called for Russian President Vladimir Putin to be assassinated.
The ministry said in a statement: “This is a public, terrorist appeal that is completely unacceptable.”
If Washington did not unambiguously condemn the remarks and take concrete measures, it “will have a further devastating effect on Russian-American relations”, the ministry added.
Relations between the superpowers are already at a low following Western sanctions against Russia over its invasion of Ukraine.
• Russia continues its onslaught on Ukraine for the tenth day
• Western sanctions ‘equivalent to declaring war’, Putin claims
• Ukrainian President Volodymyr Zelenskyy criticises NATO for refusing to impose a no-fly zone
• Britons in Russia told to consider leaving if presence ‘not essential’
• The UN says the number of refugees fleeing the fighting could rise to 1.5 million this weekend
Earlier this week, Mr Graham, 66, a Republican from South Carolina, called on Twitter for someone in Russia to “take this guy out” – referring to Putin.
White House spokesperson Jen Psaki told reporters on Friday: “We are not advocating for killing the leader of a foreign country or regime change. That is not the policy of the United States.”
Anatoly Antonov, Russia’s ambassador to the US, described the call from Mr Graham, who has been a senator since 2003, as “unacceptable and outrageous”.
Kremlin spokesman Dmitry Peskov suggested alcohol may have played a part.
“Of course, these days not everyone is managing to preserve a sober mind, I would even say a sound mind,” he said.
Mr Graham appeared unconcerned, tweeting on Saturday, after senators held talks with Ukrainian President Vlodomyr Zelenskyy that: “It is clear to me the world would be a better place if there was regime change in Russia.
“Putin is a war criminal who needs to go – at the hands of the Russian people – by any means possible.”