The European Union must not compromise with Russia over sanctions in response to the Ukraine crisis, the bloc's chief Donald Tusk said in an interview Monday.
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.
"For (the US), it's unacceptable to be more involved in the Ukrainian conflict than Europe."
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."
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 Sunday that a peace deal in eastern Ukraine signed last month was "not working".
Kiev and the West accuse Moscow of backing the separatists in the region with troops and heavy weapons, though the Kremlin denies this.
Monday 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.