Ukraine's Interior Ministry has said the number of anonymous callers phoning in bomb threats at infrastructure sites in Ukraine is increasing and that most are coming from Russia and Russia-occupied areas of Ukraine.

"There is one accent characteristic of today. There is activity on the part of Russia and its satellites in Russia-occupied territories regarding "explosive objects" in the territory controlled by Ukraine: social infrastructure sites and places where citizens gather. These are airports, and shopping centers," Interior Minister advisor Ivan Varchenko said during a briefing in Kyiv on Sunday.

Varchenko said the ministry has a clear understanding that "the messages originate from Russia-occupied territories. The messages are from there."

Varchenko said law-enforcement agencies have prepared for the bomb scares.

"We understand that the enemy will use every opportunity. Our bomb technicians are now working, checking each threat," he said.