Brigadier general Oleksiy Gromov told a military briefing on Thursday that although he did not expect Moscow to launch an attack from Belarus, Russian was training new troops on its neighbour's soil and had moved military aircraft there.
"The Kremlin ... is seeking to turn the conflict into a prolonged armed confrontation," Mr Gromov said.
Deputy defence minister Hanna Malyar, at the same briefing, warned against allowing complacency to set in after recent Russian military setbacks.
She said: “We and the world should not relax, because the ultimate goal of the Russian Federation is to conquer all of Ukraine, and then it can move on."
Ukrainian officials have portrayed the Kremlin as desperate to reverse recent military setbacks - which included a retreat from the southern city of Kherson after months of occupation - and secure victories to justify the war to the Russian public.
General Valery Zaluzhny, commander in chief of the Ukrainian armed forces, told the Economist that Russian forces were doing all they could to prevent Kyiv from striking back.
"This is why you are seeing battles along the 1,500km frontline ... they are constraining our troops in order not to allow us to regroup," the newspaper cited him as saying in an interview published on Thursday.
"The next problem that we have is, first of all, to hold this line and not lose any more ground. It's crucial ... Our troops are all tied up in battles now, they are bleeding.”
Zaluzhny said Moscow was preparing for a new assault early next year, possibly aiming for Kyiv.
The Kremlin has never fully defined the goals of its invasion, which it said was partly intended to protect Russian-speakers in eastern Ukraine.
It said last week that it was still set on securing at least the bulk of the parts of east and south Ukraine that it has declared its own, but appeared to give up on seizing other areas in the west and northeast that Ukraine has recaptured.