"The number in the calibrated foreign legion is no longer estimated at hundreds, it has already exceeded a thousand [mercenaries] in the city itself," Rogov said in a televised appearance on Russian broadcaster Zvezda.
He added that these include mercenaries from Georgia and Poland.
The official said that some 700 troops of the Ukrainian military could attempt to seize control of the Zaporozhye Nuclear Power Plant (ZNPP) in the near future.
"At least 700 people that we can see are already prepared for the assault and seizure of the nuclear power plant. In addition, we can add here armored boats and a group of several tens of thousands of people who are on the line of contact," Rogov said, adding that some 300 troops have recently arrived in the area and are currently training in forcing the Dnepr river and seizing large targets, such as the ZNPP.
Rogov cited intelligence information as saying that the troops' task is to advance to the city of Berdyansk, cut Russian forces from the land route to Crimea, and attack the ZNPP.
"In other words, to assault the nuclear power plant and advance to Berdyansk - these are the two main scenarios that are visible and obvious, and which I am afraid they will attempt to implement in the near future," the official said.
The ZNPP, located on the left bank of the Dnepr River, is the largest nuclear power plant in Europe in terms of the number of units and output. During the military operation in Ukraine, launched by Russia on February 24, the facility and the surrounding area fell under the control of the Russian forces.
In early September, an International Atomic Energy Agency (IAEA) mission led by IAEA Director General Rafael Grossi arrived at the ZNPP. The head of the Rosatom delegation and ZNPP staff led the delegation through its territory and showed sections damaged during shelling on the part of Ukrainian troops.