Russian prosecutors have asked at least two state-owned companies to provide information about possible illegal actions taken by opposition leader Aleksei Navalny, who says he believes similar requests have been sent to several other companies targeted in his anticorruption investigations.

In a letter dated April 28, a senior Prosecutor-General's Office official asked the general director of state-controlled airline Aeroflot to provide "information about any illegal actions committed" by Navalny, according to a scan of the letter that Navalny published on his website on May 11.

An unidentified source in the Prosecutor-General's Office confirmed the authenticity of the letter, according to the daily RBK newspaper, which reported on May 11 that an analogous request had been sent to state-owned oil major Rosneft.

Navalny, who previously served on Aeroflot's board of directors, built his reputation as a relentless anticorruption crusader through his investigations of alleged graft at state-owned firms in which he was a minority shareholder.

He has repeatedly accused Rosneft, headed by Kremlin insider Igor Sechin, of corrupt dealings.

The move by Russian prosecutors to ferret out potential wrongdoing by Navalny is the latest in a series of legal assaults by authorities against the opposition politician, who was a driving force behind antigovernment street protests in 2011-12.

He is currently serving two suspended sentences on financial-crimes charges that he and his supporters call retribution for his activism.

The European Court of Human Rights (ECHR) in February ruled that Russia violated Navalny's right to a fair trial in one of the cases and ordered the government to pay him 56,000 euros ($64,000) in legal costs and damages. Russia said it would appeal the decision.

Navalny wrote on his website on May 11 that he believes prosecutors have sent similar requests for information to other state-owned companies that he has investigated for corruption, including lender VTB, energy giant Gazprom, and pipeline monopoly Transneft.

RBC quoted an unidentified source with federal prosecutors as saying requests for information about alleged malfeasance by Navalny had been sent to "scores of other companies."

With reporting by RBC.ru