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"I would like to associate myself with the many condemnations that have been voiced about Ms. Savchenko's detention," the letter reads.

Noting that he "fully shares" the Minister's "outrage and concern" about Ms. Savchenko's ongoing detention and deteriorating health, Mr. Turner's message notes: "As you know, I have several times before urged Russian authorities to release all the Ukrainian prisoners illegally detained in connection with Russia's military aggression against Ukraine, as required under the Minsk agreements. Sadly, to this day, Russia has failed to implement this and almost all other conditions set forth in the agreement."

Read alsoEU ambassadors extend sanctions against Russians for 6 monthsFollowing the European Union's decision last week to extend existing sanctions against Russia, the Assembly President stressed the importance for NATO governments and parliaments to maintain a firm line. The NATO PA will "remain firm in our condemnation of Russia's aggression and its illegal occupation of parts of Ukraine's sovereign territory," Mr. Turner assures Minister Klimkin. "We will continue to urge Russia to fully abide by its commitments under the Minsk agreements and under international law."

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Read alsoU.S. urges Russia to pressure Donbas militants over attacksSince its illegal annexation of Crimea in March 2014, Russia continues to support armed separatists in Eastern Ukraine, propping up a conflict that has already killed 9,000. The NATO PA is due to hold a high-level seminar in Kyiv in June to review developments in the country and the impact of Russia's ongoing aggression.