The White House condemned what it called the "brutal" murder of Russian opposition leader Boris Nemtsov, a fierce critic of President Vladimir Putin who was shot dead in Moscow on Friday.
"We call on Russian govt to conduct prompt, impartial & transparent investigation & ensure those responsible are brought to justice," President Barack Obama`s National Security Council said in a tweet.
"United States condemns brutal murder of Boris Nemtsov," it said in another tweet.
There was no further immediate reaction from Washington, whose 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 which reports directly to Putin said "experienced" investigators had been tasked with looking into Nemtsov`s killing.
Putin`s spokesman Dmitry Peskov said the murder "bears the hallmarks of a contract killing."