Some experts expected an attack on May 9, celebrated as Victory Day in Russia and other former Soviet countries, but that day passed without much incident. However, the danger of possible active offensive operations is increasing, the ATO headquarters said.

Intelligence suggests that the militants have restocked their weapon and ammunition supplies, and they are targeting the positions of the Ukrainian armed forces in order to improve their skills for the active phase of hostilities.

The most threatening force of the enemy is its tank brigades, the headquarters said. The number of armored vehicles of the Russian-backed militants is already almost 700 units, it said.

"On the territory of Luhansk region and Luhansk the militants have created three powerful rear maintenance bases, where they store their armored vehicles. There is a lot of military equipment in Donetsk - the militants hide it in residential areas, industrial bases, and warehouses," National Security and Defense Council spokesman Andriy Lysenko said.

In addition, there are reports that the militant organizations may be planning to hold another sham referendum in the Donbas.