Kiev, Ukraine (CNN) -- Twenty Ukrainian prisoners of war have been released under the ceasefire between Kiev and pro-Russian rebels signed in Minsk, Belarus, last week, Ukraine's President Petro Poroshenko said Monday. Ukraine's counterterrorist operation press office said the freed prisoners were Ukrainian troops, telling CNN they had been exchanged for a "terrorist." Speaking in the port city of Mariupol, south of Donetsk, Poroshenko said 33 Ukrainian servicemen who had refused to surrender had also been able to leave the conflict zone. The Russian state news agency ITAR-Tass quoted the vice premier of the self-proclaimed Donetsk People's Republic, Andrei Purgin, as saying both sides had agreed to exchange "30 for 30" prisoners as a starting point.
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NEW: The European Council formally adopts further sanctions against Russia on Monday
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Ukraine and rebels confirm a prisoner release under the Minsk truce agreement
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The Kremlin says the Ukrainian and Russian presidents discussed the truce Monday
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Ukrainian President Petro Poroshenko confirms death of one soldier, one civilian
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"The sanctions aim at promoting a change of course in Russia's actions destabilising eastern Ukraine and come in the wake of the @placeholder of 30 August, which condemned the increasing inflows of fighters and weapons from the territory of the Russian Federation into Eastern Ukraine and the aggression by Russian armed forces on Ukrainian soil," he said in a statement.
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"The sanctions aim at promoting a change of course in Russia's actions destabilising eastern Ukraine and come in the wake of the European Council of 30 August, which condemned the increasing inflows of fighters and weapons from the territory of the Russian Federation into Eastern Ukraine and the aggression by Russian armed forces on @placeholder soil," he said in a statement.
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Poroshenko told staff at a metal works in Mariupol that @placeholder forces would fight to ensure the city remained part of Ukraine.
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Sporadic artillery and machine gun fire rang out in the early hours of Sunday on the outskirts of @placeholder.
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ITAR-Tass late Saturday quoted rebel officials as saying that @placeholder forces continued to shell Donetsk and rebel positions near Mariupol.
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@placeholder forces and the rebels have engaged in fierce fighting over the past week in the territory between the Russian border and Mariupol.
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A previous unilateral ceasefire declared by the @placeholder government in June broke down after 10 days.
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But he denies that @placeholder has armed and trained the rebels, or sent its troops over the border.