Economic Development Minister Alexei Ulyukayev is the most senior government official to face charges in years.

A long-serving Russian minister has been arrested on suspicion of taking a bribe over a deal involving the state-controlled oil giant Rosneft.

Economic Development Minister Alexei Ulyukayev was arrested late on Monday after he allegedly received a $2m bribe in a sting operation set by the FSB, the KGB’s main successor agency, the Investigative Committee said in a statement on Tuesday.

The investigators said Ulyukayev, the most senior government official to face charges in years, took the bribe for giving the green light to Rosneft to take part in bidding for another oil company.

Ulyukayev, who held the post since 2013 and worked in the government since 2000, denied he is guilty of receiving bribes, an investigator told a court on Tuesday.

He is a known liberal figure who has spoken against an increasing government presence in the Russian economy.

Ulyukayev had opposed Rosneft’s bidding for Bashneft, saying it was wrong for a state-owned company to take part in the privatisation drive.

President Vladimir Putin had defended the deal, saying that because Rosneft has minority foreign investors, the sale was not simply a transfer of assets from one part of the state to another.

However, Al Jazeera’s Rory Challands, reporting from Moscow, said the move involved transferring money from a government-owned entity to another, “even though it was dressed, controversially, inside Russia as being a big score for privatisation”.

“But almost nobody in Russia will believe this is purely about corruption, because this is a country where government ministers – supposed to be on fairly modest salaries – wear $40,000 watches, drive Bentleys, and live in mansions,” Challands said.

“Usually, when an arrest like this happens, it’s something else going on – a power struggle, possibly. One theory at the moment is [that] this is a power struggle that might involve the boss of Rosneft, [who is] a long-time ally of Vladimir Putin.”

Russian Prime Minister Dmitry Medvedev appointed Deputy Economy Minister Yevgeny Yelin as acting economy minister after Ulyukayev’s arrest, a spokeswoman for the prime minister’s office said later on Tuesday.