Arizona Sen. John McCain has blamed the killing of Russian opposition leader Boris Nemtsov on an “environment of impunity that Vladimir Putin has created in Russia,” joining a chorus of U.S. leaders condemning the killing.

In a statement, McCain said that he was “devastated to learn of the murder in Moscow today of my friend Boris Nemtsov,” adding that “regardless of who actually pulled the trigger, Boris is dead because of the environment of impunity that Vladimir Putin has created in Russia, where individuals are routinely persecuted and attacked for their beliefs, including by the Russian government, and no one is ever held responsible.”

The allegation is the latest in a round of hostilities between the Russian leader and the former GOP presidential candidate. Earlier, Putin had branded McCain a "war criminal" after the death of Libyan dictator Moammar Gadhafi in 2011.

"Mr McCain fought in Vietnam," Putin said, according to The Telegraph. "I think that he has enough blood of peaceful citizens on his hands. ... Mr McCain was captured and they kept him not just in prison, but in a pit for several years," he continued. "Anyone [in his place] would go nuts."

After Putin took to the pages of The New York Times earlier this year to tell Americans they should not use military force in Syria, McCain penned a tit-for-tat response in Pravda, headlined "Russia deserves better than Putin", according to the Guardian.

McCain had previously suggested that Putin was “literally getting away with murder,” in the aftermath of the shooting down of Malaysia Airlines Flight MH17, according to The Hill.

McCain was not the only U.S. politician to condemn the murder. In a statement, President Obama said “I admired Nemtsov’s courageous dedication to the struggle against corruption in Russia and appreciated his willingness to share his candid views with me when we met in Moscow in 2009. We offer our sincere condolences to [Nemtsov's] family, and to the Russian people, who have lost one of the most dedicated and eloquent defenders of their rights."