KIEV (REUTERS) - Ukraine's acting president signed a decree on Saturday declaring the appointment of a pro-Russia premier in the Crimea region illegal.

Sergei Aksyonov was appointed in Crimea this week after armed men took over the regional parliament. The presidential decree said the decision had violated the Ukrainian constitution.

It gave no details.

Aksyonov has appealed for Russia's assistance to restore calm on the Black Sea peninsula, the only region in Ukraine that has an ethnic Russian majority.

Ukraine's new leaders face a challenge to their authority in the region.