Donald Tusk, who is president of the European Council and a former Polish prime minister, made the comments in an interview broadcast on December 1 by Polish station TVN24, RFE/RL wrote.
He said France, Germany, and the United States had supported him in maintaining a cohesive response to Russia's military actions in recent years.
He said he expected another extension of the sanctions to be approved in January, for the usual six-month period.
Read alsoFT: Norway and Finland thaw relations with RussiaBut he also said the election of Donald Trump as U.S. president will also make it harder to preserve Western unity toward Russia.
"I believe that after the [U.S.] elections and Donald Trump's victory it will be harder to build such unequivocal, uniform policy of the Western world toward Russia," Tusk said.
Trump has expressed a more conciliatory approach toward Moscow.
And in France, many observers are predicting the next president will be Francois Fillon, a former prime minister who also advocates more cooperation with Russia.
