MOSCOW (Reuters) - Russia's Interfax news agency said pro-Russian rebels entered Mariupol in southeastern Ukraine on Friday but a military spokesman in Kiev said Ukrainian forces were still holding the strategic port city.

"The first (rebel) groups ... are already in Mariupol," Interfax quoted a rebel source as saying.

Andriy Lysenko, spokesman for Ukraine's National Security and Defense Council, said in Kiev: "It is not true. We have even driven them back from the positions they held before."

(Reporting by Katya Golubkova, Aleksandar Vasovic and Pavel Polityuk, editing by Timothy Heritage)