Imperial Energy, the London-listed oil explorer accused by Russia of overstating reserves, rejected a bid by Gazprombank to buy as much as 25 percent of the company's stock.

Gazprombank, the banking unit of Gazprom, offered to buy the shares at a discount to their Nov. 1 price, Imperial said Thursday, adding that it would seek other ways of working with Gazprombank. "Both groups take the view that there is room for mutual cooperation between them in the petroleum industry of Russia in other ways," Imperial said. "This is intended to be pursued in due course and as may be appropriate."

In July, the Natural Resources Ministry's environmental watchdog called Imperial's reserves statements "substantially overstated and misleading," in a letter to Britain's Financial Services Authority, which regulates financial markets.

Imperial, whose reserves were audited by U.S. company DeGolyer & MacNaughton, has denied inflating its reserves.

Gazprom in the past year took control of oil and gas projects from Shell and BP in Russia after complaints of regulatory violations by the state.

Analysts Artyom Konchin and Tatyana Kapustina of Aton Capital said in a note to investors earlier this month that a strategic partnership with Gazprombank would "diminish the risks stemming from the company's long-lasting conflict" with the Natural Resources Ministry.