BRUSSELS—The European Union has "strong concerns" about a decree passed by the Russian government threatening new tariffs on Ukrainian exports, European Commission President Jose Manuel Barroso said in a letter to Russian President Vladimir Putin.

The decree signed Sept. 19 by Russian President Dmitry Medvedev, which will impose Russian tariffs if Ukraine lowers its import tariffs on goods from the EU before 2016, undermines a cease-fire deal agreed in Minsk by Kiev, Moscow and the EU, Mr. Barroso wrote.

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