"Thirteen Russian aircrafts landed at the airport of Gvardeyskoye (near Simferopol) with 150 people in each one," Sergiy Kunitsyn, the Ukrainian president's special representative in Crimea, told a local television channel, adding the air space had been closed.

It was not immediately clear if Russia had the right to use the base or send additional troops there under its agreements with Ukraine.

Earlier, US President Barack Obama had expressed deep concern over reported Russian military activity inside Ukraine and warned that there will be costs for any intervention.

Obama also said any such Russian move would represent a profound interference in matters that must be determined by the Ukrainian people.


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