Pro-Russian separatists have seized part of a hospital building in the eastern Ukrainian city of Donetsk, Unian reports.

A spokesman for the regional administration said the separatists apparently wanted to use the clinic to provide treatment for injured fighters. 

Ukraine's Border Guard Service, meanwhile, said eight crossings on the border with Russia in Donetsk and neighboring Luhansk region were temporarily closed on June 6 after heavy fighting.

Among those was the border crossing near the town of Marynivka, some 95 kilometers east of Donetsk, where the agency said 15 separatists were killed and five border guards wounded in overnight clashes. 

Ukraine's government has said separatist fighters are crossing the border from Russia.

Russia's Foreign Ministry said it was "outraged" by the closure of the border crossings.