US President Barack Obama condemned what he called the "brutal" and "vicious" murder of Russian opposition leader Boris Nemtsov, a fierce critic of President Vladimir Putin who was shot dead in Moscow on Friday.
"We call upon the Russian government to conduct a prompt, impartial, and transparent investigation into the circumstances of his murder and ensure that those responsible for this vicious killing are brought to justice," Obama said in a statement.
"Nemtsov was a tireless advocate for his country, seeking for his fellow Russian citizens the rights to which all people are entitled."
"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," added Obama, calling Nemtsov a "dedicated" and "eloquent defender" of rights.
Washington's ties with Moscow are at their lowest ebb since the Cold War over eastern Ukraine, where pro-Russian rebels are waging what the United States has labeled "a landgrab."
Moscow denies backing the rebels.
Nemtsov's killing in central Moscow comes ahead of a major opposition rally this weekend.
A committee that reports directly to Putin said "experienced" investigators had been tasked with looking into Nemtsov's murder.
Putin's spokesman Dmitry Peskov said the murder "bears the hallmarks of a contract killing."
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