GENEVA (AFP) - The referendum to be held in the Crimean peninsula on joining Russia violates Ukraine's constitution, the Organization for Security and Cooperation in Europe's chairman said on Tuesday.

"In its current form the referendum regarding Crimea scheduled for March 16, 2014, is in contradiction with the Ukrainian constitution and must be considered illegal," OSCE Chairman Didier Burkhalter was quoted as saying in a statement.

He ruled out an OSCE observation of the referendum in Crimea, where pro-Russian forces took control after pro-European protesters ousted the Kremlin's ally from power in Kiev.

The strategic region's self-declared rulers are now recruiting volunteers to fight Ukrainian soldiers while the Russian government sent lawmakers a draft bill that would simplify the Kremlin's annexation of Crimea after Sunday's vote.

But Kiev rejects the referendum and is appealing to Western powers for both diplomatic backing and pressure on Moscow to release its troops' stranglehold on the mostly Russian-speaking region.