By Mick Krever, CNN

Pro-Russian separatists will “liberate” the eastern Ukrainian city of Mariupol, Russia's Permanent Representative to the OSCE, Andrey Kelin, anticipated in an interview with CNN’s Christiane Amanpour Friday.

Ukrainian forces are working to fortify the city; they claim that Russian intelligence groups have been spotted in the area.

Mariupol is “the second-biggest city in Donetsk Oblas, probably, and I believe that they are going to liberate,” Kelin said.

The majority of the area’s population “are ethnic Russians, although they are bearers of Ukrainian passports.”

So the fact that the city should fall from Ukrainian support, he said, “is natural, absolutely.”

Friday saw Ukraine and pro-Russian separatists agree to a ceasefire in eastern Ukraine. But NATO Secretary General Anders Fogh Rasmussen told Amanpour that there are several thousand regular Russian forces operating in Ukraine.

Amanpour asked Kelin whether Russia would withdraw these forces.

"There are no Russian forces in Ukraine, and this is absolutely sure.”

“But at the same time I do not deny at all that there might be volunteers over there.”