"We have reasons to suspect - we have been receiving such information in the last several hours - that the risk of a direct intervention (by Russia's military in Ukraine) is for sure higher than it was several days ago," Tusk told at a news conference, Censor.NET reports citing Reuters.
Russia has been conducting exercises near the border with Ukraine, raising concerns on the ground and in a number of Western capitals.
Moscow has also called an emergency meeting of the United Nations' Security Council over what it described as a "humanitarian situation" in Ukraine.
"If (it) were to come to a direct intervention of Russian forces in Ukraine than this would obviously be a qualitatively new situation and in my opinion nobody has a good, unequivocal answer today how the Western community should react to that," Tusk said.
Tusk also said that sanctions imposed by the European Union on Russia will be costly for Europe's economy, but a lack of reaction to Russia's actions during the Ukraine crisis would have the most "catastrophic" consequences.
On Tuesday, Russian President Vladimir Putin ordered his government to prepare retaliatory measures against the latest round of Western sanctions.