MOSCOW  Already jittery investors were alarmed on Thursday when a Norwegian cellphone company announced that a Siberian court had seized its multibillion-dollar investment in a Russian joint venture and would turn it over to a company thought to be allied with a Russian oligarch.

The decision signaled an escalation in a long-running dispute between the Norwegian company, Telenor, and the Alfa Group, an alliance of Russian businessmen that was also at the center of a separate fight with the British oil giant BP last summer. That dispute also shook faith in the Russian market.

Russia’s stock market fell on the news of the asset seizure.

Telenor has accused the Alfa Group, whose principal partner is Mikhail M. Fridman, of filing groundless lawsuits to gain control of Vimpelcom, the cellphone company that they own jointly. One such lawsuit prompted the ruling Wednesday.

A judge in the Siberian city of Omsk ordered court bailiffs to seize Telenor’s 29.9 percent share in Vimpelcom. The judge had earlier ordered Telenor to pay a fine of $1.7 billion after Alfa accused Telenor of obstructing the expansion of Vimpelcom into Ukraine to protect other Telenor businesses in that country. Telenor denied the accusation and refused to pay the fine. The shares were seized in lieu of the payment.