"Nemtsov was a tireless advocate for his country, seeking for his fellow Russian citizens the rights to which all people are entitled," Obama said in a statement on Friday.

"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," he said.

"We offer our sincere condolences to his family, and to the Russian people, who have lost one of the most dedicated and eloquent defenders of their rights," Obama said.

Killing of the Russian opposition leader is reflective of the lawlessness that prevails in Russia, said Congressman Ed Royce, Chairman of the House Foreign Affairs Committee.

"The US must strongly support all those brave individuals who refuse to be silenced by fear and who knowingly risk their lives to bring freedom to Russia," he said.

An unidentified attacker in a car shot Nemtsov four times in the back as he crossed a bridge in view of the Kremlin early today. He died hours after appealing for support for a march on Sunday in Moscow against the war in Ukraine.

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