A man believed to be strapped with explosives has taken nine adults and two children hostage in a post office in the Ukrainian city of Kharkiv, police have said.

The unidentified man is calm, has not made any demands and is in regular contact with police through the telephones of the hostages, regional police chief Oleg Bekh told 112 news channel.

Television footage showed police and parked police cars outside a white-and-yellow two-storey building in the north-eastern city. The area has been closed off to traffic.

"We are trying to do everything to maintain communication with him and to do everything that is necessary to ensure the people are released," Mr Bekh said.

He added the man in the post office was concerned about the recent prisoner exchange between the Ukrainian authorities and pro-Russian separatists and thought more prisoners should have been released. Police did not know what the hostage-taker wanted.

"His demands are unknown to us," police spokesman Yaroslav Trakalo said.

Ukraine swapped hundreds of prisoners with the separatists on Wednesday in the biggest such exchange since the outbreak of a conflict in the eastern Donbass region that has killed more than 10,000 people.

According to the terms of the deal, Kiev was to hand over 306 prisoners to the rebels and receive 74 prisoners in return.