Russian President Vladimir Putin has invited Greece's new prime minister to Moscow in May, as Athens and Berlin publicly traded barbs over European Union policy on Ukraine.

Alexis Tsipras expressed his intention to attend May 9 celebrations for the World War II victory over Nazism, his office said.

Greece has traditionally close ties with Russia, and Tsipras' new left-wing government last week claimed credit for blocking new EU sanctions on Moscow over fighting in Ukraine.

In an interview before a NATO defense ministers' meeting in Brussels on Thursday, German Defense Minister Ursula von der Leyen expressed annoyance at Greece's position.

Greek Defense Minister Panos Kammenos responded, calling her comments "unacceptable and extortionate."

He added: "Greece was always on the side of the Allies when they repulsed German (WWII) occupation troops."