STORY HIGHLIGHTS

(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.

Moscow has consistently denied allegations by Kiev and the West that Russia has troops in Ukraine, and that it has armed and supported the rebels.

On Wednesday, Ukrainian President Petro Poroshenko said some 70% of the Russian troops believed to have been in Ukraine had withdrawn back across the border, according to the national news agency Ukrinform.

Information released by Ukraine's National Security and Defense Council on Thursday indicated that the rebel forces control a strip of eastern Ukraine running from the city of Luhansk down to the Sea of Azov.

'Very fragile' ceasefire

The truce was signed Friday after talks in Minsk, Belarus, between representatives of Ukraine, the rebels and Russia.

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NATO: 1,000 Russian troops in Ukraine