WASHINGTON (AFP) - The United States expressed concern on Friday over Russia's "unprecedented" media censorship, including shuttering opposition news websites amid the East-West standoff over Ukraine, and called for the Kremlin to respect press freedom.

State Department deputy spokeswoman Marie Harf cited this month's ouster of the chief editor of Lenta.ru, one of Russia's most popular news websites, after a state watchdog criticized its interviews with Ukrainian ultra-nationalists.

"The United States is deeply troubled by the rapidly shrinking space for independent and free media in Russia," said Ms Harf.

"In the last year, the Russian government has passed laws imposing unprecedented censorship and restrictions on media and online publications." Three major opposition websites have also been blocked, as has the popular blog of Kremlin critic Alexei Navalny.

"These developments effectively stifle the expression of alternative views, restrict the space for independent discussion and facilitate the spreading of government-controlled narratives that are patently false," Ms Harf said.

Russia's media crackdown comes as President Vladimir Putin stares down Western powers, with state media waging a propaganda war backing intervention in the Ukrainian crisis and the Kremlin's incursion into Crimea after a populist uprising in Kiev last month.

Ms Harf noted that the US administration supports free speech rights to all regardless of their political views.

"These are rights Russia has enshrined in its own constitution," she added.

The Internet has become practically the only outlet for opposition political views in Russia.

Russian Internet providers were blocking access to the blacklisted websites, though they were still accessible through Internet providers outside Russia.

Tips on how to get around the ban were circulating on social media.