Russia's media regulator has blocked a website backed by former oil tycoon and Kremlin critic Mikhail Khodorkovsky, the latest media site to be blacklisted by the agency.

Roskomnadzor's decision to add MBK Media to its blacklist, less than four weeks before Russia's presidential election, follows a request from the Prosecutor-General's Office.

The agency did not provide further details.

MBK said in a February 21 statement that the organization had not received any notice from Roskomnadzor about the blacklisting.

The site, which publishes a mix of news and commentary, is funded by Khodorkovsky, who was once Russia's richest man until he ran afoul of the Kremlin in the early 2000s. He was imprisoned for more than a decade on charges his supporters said were trumped up. His oil company, Yukos, was dismantled, its largest assets sold off to state oil giant Rosneft.

Since being pardoned by President Vladimir Putin in 2013, Khodorkovsky has lived in Europe and funded opposition groups, such as Open Russia.

Earlier this month, Roskomnadzor blocked the website of anticorruption crusader Aleksei Navalny, who has been barred from challenging Putin in next month's election.

Putin, who has effectively been in power since 1999, is widely expected to win a new six-year term.

Roskomnadzor late last year began blocking access to websites of organizations deemed "undesirable" by the Justice Ministry under a 2015 law aimed at restricting the activity of organizations the Kremlin accuses of fomenting political dissent.

Authorities in December 2017 blocked access to several websites of Open Russia, a civil-society nongovernmental organization founded by Khodorkovsky.

Amnesty International denounced the move, saying that Russian authorities "are clearly targeting Open Russia in a bid to suffocate dissent and pluralism in the Russian media and expunge Khodorkovsky's presence in Russian politics and society."