KIEV (Reuters) - Ukrainian authorities detained a former lawmaker and the deputy chief of state energy firm Naftogaz in a case related to the embezzlement of $17.3 million through selling uranium concentrate at inflated prices, prosecutors and the anti-corruption bureau said on Friday.

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The anti-corruption bureau NABU suspects former lawmaker Mykola Martynenko of benefiting through an Austria-registered company from the uranium sales to a state-owned enrichment plant in 2014, at a time when he was the head of the energy committee in parliament. He says the case is politically motivated.

Sergiy Pereloma, first deputy chairman of state-run Naftogaz, was also detained, suspected of being an intermediary. His lawyer denied his involvement. Naftogaz said the matter related to activities before he joined the company.

“The investigation found that the amount of losses inflicted on the state is more than $17 million dollars,” chief anti-corruption prosecutor Nazar Kholodnytskyi told a news conference.

Kiev needs to prove to its Western backers that it has become serious about tackling corruption, which it must do as part of a $40 billion bailout rolled out after street protests in 2013/2014, the annexation of Crimea and the outbreak of separatist fighting in eastern Ukraine.

The International Monetary Fund has previously criticized Kiev for not prosecuting and convicting corrupt high-level officials. In April, the IMF said Ukraine’s anti-graft drive had yet to achieve “concrete results”.

Martynenko is the most high-profile person to be detained for corruption since Roman Nasirov, the head of the fiscal service, who is now under house arrest over the alleged embezzlement of around $75 million. Nasirov denies wrongdoing.

“This case is political and is not backed by any evidence,” Martynenko said in a video posted on the website of his People’s Front party.

Pereloma’s lawyer Igor Cherezov told Reuters: “He is accused of collusion with Martynenko.”

Cherezov said his client was “extremely surprised”. “He has as much involvement in the case as you or I,” he said.

Naftogaz’s spokeswoman said the accusations against Pereloma related to his activities before he joined the company, and that Naftogaz was willing to cooperate with the investigation.

Naftogaz has undergone Western-backed reforms since 2014 but the European Bank for Reconstruction and Development has warned that these changes are in danger of unraveling.