Earlier in the day, the US broadcaster reported that its journalists traveled to Crimea by train from Moscow and talked with local residents.
"Visiting... Crimea from the territory of the Russian Federation is a violation of the legislation of Ukraine. Legal liability is provided for violators. In particular, foreigners also face a ban on entry into Ukraine for such actions," Nikolenko said on social media.
The competent Ukrainian authorities are now establishing the circumstances of the journalists' visit to Crimea, the spokesman added.
Crimea reunified with Russia following a referendum held in March 2014, in which nearly 96% of Crimeans voted for accession to Russia. Ukraine still considers Crimea to be its own territory that was illegally occupied by Russia. Moscow has repeatedly stated the decision made by the Crimean people was in full compliance with international law and the UN Charter.