Tusk said that a failure to uphold the sanctions until Ukraine regains control of its border with Russia could strain relations with the US.
He added that it was either "naivety or hypocrisy" to trust that President Vladimir Putin was acting in a spirit of goodwill.
"Partial implementation would be very risky for Ukraine... if Europe doesn't maintain existing sanctions, it would be a critical moment in transatlantic relations," the president of the European Council said.
He added: "I can't accept the argument that we have to believe in Russian goodwill. When I hear we have to believe in President Putin's or the separatists' goodwill, I know it's either naivety or hypocrisy. The comparison with appeasement applies."
More From This Section
Tusk's comments came in an interview published by several European newspapers including Britain's Guardian.
He also confirmed that he would not attend an event in Moscow to mark 70 years since the end of World War II.
Ukrainian President Petro Poroshenko said yesterday that a peace deal in eastern Ukraine signed last month was "not working".
Today marks the first anniversary of a referendum in Crimea on splitting from Ukraine which set the stage for Russia's annexation of the peninsula and inspired the separatist movement in Ukraine's east.