(CNN) -- Russia still has about 1,000 troops inside eastern Ukraine, a NATO military officer said Thursday, a week after Ukraine's government and pro-Russian rebels agreed to a ceasefire aimed at ending months of conflict. NATO also sees 20,000 more Russian troops aligned along the border, the NATO officer told CNN. He was not named according to standard practice in the organization. NATO believes that these numbers amount to a large and effective military force, the officer said. In addition to the troop numbers, NATO continues to see sophisticated Russian military equipment in Ukraine. NATO remains concerned and urges Russia to engage with the international community and Ukraine to find a political solution to the crisis, the officer said.
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NEW: Obama: The U.S. will join the European Union in implementing sanctions
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Russia has 1,000 soldiers and advanced military equipment in Ukraine, a NATO officer says
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Ceasefire in eastern Ukraine is "very fragile," Russian Foreign Ministry spokesman says
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He cites "mounting evidence" of Kiev government strengthening its military presence
@placeholder officials will review the implementation of the ceasefire before the end of the month, he said, after which the sanctions could be amended or lifted.