Armed pro-Russian activists stand guard at the entrance of the seized regional government headquarters in Luhansk, eastern Ukraine, May 5, 2014. Four Ukrainian paramilitary police were killed in fighting on Monday with pro-Russian separatists near the rebel stronghold of Slaviansk, the Interior Ministry said, in renewed violence Kiev is struggling to stop across the east. [Photo/Agencies]

It increased fears that trouble could spread to the capital in the approach to Friday's celebrations of the Soviet victory in World War Two, an event that could kindle tensions over Kiev's relations with its former communist masters in Moscow.

Over 40 people were killed in Friday's clashes, the worst since pro-Moscow president Viktor Yanukovich fled to Moscow in February amid protests by Ukrainians demanding closer ties to Europe. Most were pro-Russians killed when the building they occupied was set ablaze by petrol bombs.

It is not clear who started the fire, but Moscow accuses Kiev of inciting violence.