Yefim Fiks, a spokesman for the de facto Crimean parliament, told the Russian news agency Ria Novosti on Friday that the move was necessary “to protect the local population from the crazy antics of the current Ukrainian government”.
He claimed that “everything can be expected” from the authorities in Kiev.
“Even if these provocations do not come from the authorities, inadequate radicals can do it,” Fiks said.
Andriy Demchenko, the spokesman for Ukraine’s border guard service, told Reuters news agency that the measure was “mere propaganda”.
“It is more likely a propaganda move by Russia to demonstrate strengthening security, separating from Ukraine and to make sure that, as they say, sabotage groups won’t be able to cross the border in future,” he said.
“I hope that neither this fence nor other actions by occupants will lead to limitation of Ukrainian citizens’ right to visit the peninsula.”
The European Union regards Russia’s annexation of Crimea as a violation of international law and insists on the territorial integrity of Ukraine.
The conflict between the Moscow-backed separatists and the Ukrainian army in the east of the country has killed more than 10,000 people since 2014, according to the United Nations.
In the period until shortly before Christmas, 104 dead have been counted so far this year.
In the area of conflict, a truce has been called for on Saturday starting at 0:01am (22:01 GMT).
Yevgen Marchuk, the Ukrainian ambassador to the peace talks with the rebels, said: “We stressed that this ceasefire must be used as much as possible for the repair of critical infrastructure facilities that are damaged. Unfortunately, there are several such facilities.”
INSIDE STORY: What triggered the military confrontation in Black Sea? (25:31)
The ceasefire does not have a time limit and is not just until Orthodox Christmas on January 7, the Organization for Security and Cooperation in Europe (OSCE) said on Friday.
Welcoming the truce deal, German Chancellor Angela Merkel and French President Emmanuel Macron pressed Russia in a joint statement on Friday to release Ukrainian sailors captured over a month ago in time for the new year and Orthodox Christmas.
They also vowed to keep up pressure to implement a 2015 peace deal for eastern Ukraine.
The long-simmering conflict between Russia and Ukraine that started with Russia’s annexation of Crimea escalated on November 25 when the Russian coastguard fired upon and seized three Ukrainian naval vessels and their crews.