Paweł Soloch, head of Polish President Andrzej Duda's National Security Bureau, was speaking after Duda met his country's top military commanders on Wednesday to discuss the build-up by Moscow.
Soloch told reporters: "We are dealing with a concentration of Russian troops that has been unknown for many years … not only in Crimea, but also in the vicinity of Lugansk and Donetsk. This is a very serious show of force. These are units with combat capabilities."
He added: "All efforts should be made to avoid an escalation or direct confrontation despite the fact that this escalation has already taken place in practice."
"It’s about stopping this process so there is no new aggression against Ukraine," Soloch said.
Ukrainian Foreign Minister Dmytro Kuleba said on Tuesday that Russia would soon have more than 120,000 troops on his country's border. Kuleba called for new Western economic sanctions to deter Moscow from "further escalation."
During a videoconference of European Union foreign ministers on Monday, Poland's top diplomat, Zbigniew Rau, called for the EU to "take concrete steps to support Ukraine" and prevent a "further escalation of tension," the Polish foreign ministry said.
At the same time, he urged the preparation of "possible response scenarios should Russia decide to escalate."
Rau last week discussed the Russian military build-up near Ukraine with US Secretary of State Antony Blinken.