Russian police have raided the Moscow offices of the oil giant BP and its joint venture TNK-BP.
The searches have renewed fears that the Russian government wants greater control of foreign-owned energy assets.
There is market speculation that the Kremlin would like the state-owned gas firm Gazprom to buy the stakes of BP's Russian partners in TNK-BP.
Last year, TNK-BP was forced to sell its stake in the Kovykta gas field at a discount price to Gazprom.
In 2006, Shell was forced to cede control of its Sakhalin gas field to Gazprom.
"We will co-operate with the authorities but we do not know what they were looking for or why they were there," a BP spokesman said.
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