The Russian government this week blocked access to prominent websites that have been critical of President Vladimir Putin, amid heightened tensions in neighboring Ukraine. As Reuters reports, the Kremlin ordered national internet providers to block access to the blog of Alexei Navalny, a leading Putin critic, as well as a news site operated by fellow opposition figure Garry Kasparov, a Russian chess champion. The government also blocked access to two other independent news sites, Daily Journal and Grani.ru, under a controversial new law that went into effect on February 1st.

The law allows Russian authorities to block any sites that call for unauthorized public demonstrations or publish "extremist" material. The government justified Thursday's crackdown by arguing that the targeted sites promoted "illegal activities and participation in public events held in violation of the established order." Thursday marked the first time that authorities have used the law to block independent news sites.

The crackdown continues

The announcement came one day after the editor of Lenta.ru, Russia's most popular independent news site, suddenly stepped down under apparent pressure from the Kremlin. The editor, Galina Timchenko, was replaced with a more Kremlin-friendly editor after the site published an interview with a Ukrainian nationalist leader. After the move was announced, 79 of the site's 84 staff members signed a letter of protest, saying Timchenko stepped down under "direct pressure" from the government, and 39 resigned.

This week's crackdown comes at a time when Putin and leaders in Europe and the US are grappling over the future of Ukraine, where a transitional coalition government is slowly taking form following the ouster of embattled President Viktor Yanukovych. Russia has refused to acknowledge the new government, arguing that it was put in place under a coup, and has stationed troops in the southeastern peninsula of Crimea. A referendum to secede from Ukraine and join the Russian Federation will be held in Crimea on Sunday.

Putin has gradually tightened control over Russian media outlets over the past several years, consolidating control over a landscape now dominated by state-owned companies. After widespread protests broke out in Ukraine last year, the Kremlin abruptly dissolved one of Russia's most respected news outlets, replacing it with a state-owned conglomerate. In January, the founder and CEO of VKontakte (VK), Russia's largest social network, suddenly stepped down and transferred control of the company to a prominent Putin ally.