Secretary of State Antony Blinken said Saturday the United States was "deeply troubled" by Russia's growing intolerance of free speech, on the sixth anniversary of opposition politician Boris Nemtsov's murder.

Nemtsov, a vocal opponent of President Vladimir Putin, was shot and killed on a Moscow bridge near the Kremlin on February 27, 2015.

"As we remember Nemtsov, we reaffirm our unwavering commitment to human rights and fundamental freedoms," Blinken said in a statement.

"We remain deeply troubled by the Russian government's growing intolerance of all forms of independent expression."

Several thousand Russians along with Western diplomats gathered in Moscow to mark the anniversary, laying flowers at a makeshift memorial at the spot where Nemtsov was felled with four bullets.

In 2017, a court found a former security force officer from Chechnya guilty of Nemtsov's murder and sentenced him to 20 years in prison. Four other men were found guilty of involvement in the killing.

But the slain opposition politician's family and supporters insist the authorities have failed to bring the masterminds to justice.

Blinken paid tribute to Nemtsov as a man who "dedicated his life to building a free and democratic Russia."

"Those who would speak out in defense of their freedoms and democracy in Russia continue to be targeted for attack and assassination," he said.