Russia has warned again that it reserves the right to intervene in defence of ethnic Russians it says are under threat in eastern Ukraine.

The Foreign Ministry said in a statement issued today that authorities in Ukraine have lost control over the country and are unable to provide basic public security.

Russian officials pointed to clashes overnight on Thursday in the eastern Ukrainian city of Donetsk as evidence of growing instability. One person died and 29 were injured in violence between pro-Russian and pro-Ukrainian activists.

Troops under apparent Russian command have already deployed widely across Ukraine's southern province of Crimea, which is to hold a referendum on Sunday on seceding and merging with Russia.

Ukraine has accused Russia of a military invasion, and the West is threatening Moscow with sanctions.