In remarks to parliament, Khomchak also accused pro-Moscow separatists of systematically violating a ceasefire in the conflict in eastern Ukraine agreed in July 2020.
Ukraine and Russia have traded blame for a spike in violence in the conflict, which Kyiv says has killed 14,000 people since 2014.
Government forces and pro-Russian separatists accuse each other of breaching the ceasefire, and lawmakers say 26 Ukrainian servicemen have been killed so far this year, including four killed by snipers last week.
"The Russian Federation continues its aggressive policy towards Ukraine," Khomchak told parliament.
Khomchak said Russian troops from different regions had been assembling near the borders of Ukraine under the guise of maintaining combat readiness and preparing for exercises, "carring out a gradual build-up of troops near the state border of Ukraine."
"An additional concentration of up to 25 tactical groups is expected, which, together with the already deployed forces near the state border of Ukraine, poses a threat to the military security of the state," Khomchak said.
He added that Moscow holds 32,700 troops in Crimea, annexed by Russia in 2014, while its officers command 28,000 separatist servicemen stationed in temporarily occupied territories in eastern Ukraine.Ukraine, Western countries and NATO accuse Russia of sending troops and heavy weapons to prop up separatists. Moscow says it only provides political and humanitarian support to the rebels and says Russians fighting in Ukraine are volunteers.