Sibneft denies any wrong-doing

The company has been presented with a bill for $420m, Reuters said, just two days after getting a demand for $1bn.

Sibneft - which is controlled by Chelsea football club owner Roman Abramovich - has denied any wrongdoing.

Last year rival oil firm Yukos, which Sibneft nearly merged with, was presented with a $3bn tax demand.

Yukos' former chief executive Mikhail Khodorkovsky is currently in prison facing charges of fraud and tax evasion.

New focus

Tuesday's $1bn demand was the first time Sibneft has been faced with a bill for back taxes.

Stephen O'Sullivan at United Financial Group in Moscow said the new tax demands were "potentially quite worrying for investment in Russia."

"Companies that seem to have settled their tax affairs legitimately several years ago (are having) them raked up again by different bits of government," he told the BBC's World Business Report.

Sibneft told Reuters on Thursday that it had not broken any tax rules.

"We can confirm having received an additional claim. We keep saying that all our activities were absolutely legal," an unnamed Sibneft official told the news agency.

"We are going to present our arguments to the tax ministry, work in a spirit of cooperation with them and cite the previous check by the Audit Chamber, which found no wrongdoing."

The $420m bill was present to the firm's Noyabrsk production unit in Siberia for the 2000/01 period.