According to Kirby, these actions are unacceptable and run counter to the Minsk agreements.
He said that any attempt by the militants to occupy more Ukrainian territory would lead to "an increase in the price" of such Russian actions, that is, to a strengthening of sanctions.
The situation has recently been tense on the front line in the east of Ukraine. Russian-backed militants are constantly firing on the positions of the Ukrainian troops and settlements in order to provoke return fire from the Ukrainian side, the Ukrainian military says.
Some analysts say that the militant forces and Russian troops in the east of Ukraine are preparing for a new offensive to try to increase the amount of territory they control in the Donbas.
As UNIAN reported earlier, the permanent representatives of the EU member states agreed to extend economic sanctions against Russia until January 31, 2016. It was also agreed that the sanctions to invest in the Crimea would be extended for another year.