Officials believe that even if the Ukrainian forces are encircled and cut off from supplies they will fight with the same determination as that shown by their comrades in Mariupol.
The number of Ukrainian forces thought likely to be trapped is unknown, but about 300,000 civilians are in the area.
During a briefing with journalists, the official was asked by The National if reports of Ukraine being surrounded in Donbas were accurate.
“The Russians have now reorientated and moved their troops around in order to advance … to incrementally gain the territory in order to close that pocket,” he replied. “It’s our view that once they have joined up, no further aid will get into that group.”
The Russian capture of the area will happen “incrementally over time” he said, and “that pocket is a more achievable goal for the Russians”.
With Ukraine forces cut off it would be difficult to get arms and ammunition into the area through the “rat runs” currently used for supplies.
But being surrounded did not necessarily mean the Russians would be victorious.
“Once they’re encircled I’d emphasise that does not mean defeat for the Ukrainians. They will fight on and they have shown that they’ll fight on through their efforts in Mariupol.”
The cut-off troops would also perform a “vital function” because they would “degrade the Russian forces and create time for the remainder of Ukrainians to develop further defensive positions”, the official said.
Once the Russians had taken the pocket they will then focus on the bigger prize of Kramatorsk, the official said.
“It has always been the case that Russia has more mass and more capability than their Ukrainian counterparts.”
But he warned the current Russian offensive would be relentless. “It’s inevitable that Russians having secured the Severodonetsk pocket they will then try and move on Kramatorsk … that that will be the next goal for them.”
He said the Russians were still losing lots of drones and were attempting to mobilise men aged over 40 to fill the ranks of the estimated 15,000 dead they have suffered.
If the Russians take too long in taking the Severodonetsk pocket, “their forces will suffer further losses and if it was getting to the winter months, that's a different proposition,” he said.