Washington (AFP) - Controversial Russian media mogul Mikhail Lesin, who helped launch the English-language television network RT, has been found dead at a Washington hotel. He was 57.

RT, formerly known as Russia Today, said the former minister of media affairs died of a heart attack.

"Lesin died. It's impossible to believe this," tweeted Margarita Simonyan, editor-in-chief of RT, which is state funded.

Police in Washington said Lesin was found unresponsive early Thursday downtown, at a location ABC News identified as the Dupont Circle Hotel.

"A ruling on the cause and manner of death is pending further investigation," a statement added on Saturday.

A controversial figure, Lesin had been accused of limiting press freedom in Russia.

In a terse statement, Russian President Vladimir Putin's spokesman Dmitry Peskov told reporters that the "president highly values the enormous contribution Mikhail Lesin made helping establish Russian media."

US officials notified the Russian Embassy of Lesin's death and authorities from both countries are trying to determine the circumstances in which he died.

Lesin was Russia's minister of press, television and radio between 1999 and 2004, and later served as a Kremlin aide.

In 2013, he became head of Gazprom-Media Holding, the media arm of state energy giant Gazprom, and oversaw the work of Russia's top liberal radio station Echo of Moscow.

Lesin resigned a year later, citing family reasons.

In a recent interview, the former editor of state news agency RIA Novosti, Svetlana Mironyuk, claimed Lesin was one of two people behind her sacking in 2013.

- 'A state man' -

Mironyuk told the Russian edition of Forbes she was let go after she became a student at the University of Chicago Booth School of Business, with Kremlin officials telling her a media executive of her stature should not study in the United States.

Republican Senator Roger Wicker of Mississippi called for a probe into Lesin last year on suspicion of money laundering and corruption.

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He allegedly amassed millions of dollars in assets in Europe and the United States while working for the government, including $28 million in real estate in Los Angeles.

"That a Russian public servant could have amassed the considerable funds required to acquire and maintain these assets in Europe and the United States raises serious questions," Wicker wrote.

It was unclear whether the FBI had actually opened an investigation.

In 2014, Lesin told Forbes he found it acceptable that most television channels in Russia were state-controlled.

"I am a state man," he said.