MOSCOW (AP) - Two Russian industrial giants and oil major BP have reached a deal settling a long-running dispute over the Sidanco oil company that had worried foreign investors in Russia's lucrative and murky oil industry.

Under a memorandum of understanding announced Thursday, BP will regain influence in a prized Siberian oil field that was bought by Tyumen Oil in November 1999 at an auction BP claimed was illegal. The Chernogorneft field had been part of Sidanco, in which BP has a 10 percent stake and management control.

The deal had prompted BP to threaten to pull out of Russia, despite a half-billion dollar investment in the country. The soured investment had come to symbolize the problems faced by foreigners who invest in Russia.

Russian industrial holding Alfa Group and U.S.-based Access/Renova announced Thursday that they had jointly bought a 44 percent stake in Sidanco for dlrs 640 million from another Russian industrial giant, Interros.

The stake will be held by Tyumen Oil, an Alfa subsidiary. But the arrangement also returns the Chernogorneft unit to Sidanco, which again gives BP influence in Chernogorneft.

"BP welcomes the return of Chernogorneft assets to Sidanco," Peter Henshaw, BP's spokesman in Russia, told a news conference. "This is a long and difficult course we have steered."

The deal makes Tyumen Oil Russia's third-largest oil producer, the Moscow Times reported Friday. It was also a boost for Alfa Group and its head Mikhail Fridman.

Fridman and Interros head Vladimir Potanin were both among a coterie of powerful tycoons who amassed fortunes in often shady privatization deals in the 1990s.

After the 1999 Chernogorneft deal, a U.S. lawmaker called for an inquiry of reports that Tyumen pushed Chernogorneft into bankruptcy to obtain its oil fields at a fraction of their value. BP complained that the Russian government approved the bankruptcy auction despite its offer to settle Chernogorneft's debts.

The United States then blocked dlrs 500 million in loan guarantees to Tyumen, saying it was not in U.S. interest.