Armed men seized the capital of a Russia-backed separatist republic in eastern Ukraine amid a power struggle between two top officials.

The takeover of Luhansk on Tuesday followed a decision by Igor Plotnitsky, the head of the breakaway Luhansk People's Republic, to fire interior minister Igor Kornet the night before.

Luhansk state television showed masked men with assault rifles and heavy machine guns blocking the entrances of administrative buildings and streets in the centre of Luhansk on Tuesday, reportedly on the orders of the interior ministry. One of them wore the blue uniform the interior ministry's Berkut unit, a police force specialising in putting down riots.

The armed men had ignored Plotnitsky's command to leave, Russian independent newspaper Novaya Gazeta reported.

A video on Tuesday evening showed what appeared to be dozens of military vehicles entering Luhansk from the direction of the neighbouring Donetsk separatist republic, although a Donetsk official denied it was intervening.