Ukraine on Monday barred the powerful Moscow mayor from entering the country in the future for reportedly suggesting that it should cede a key city to Russia.

The move further strains already tense relations between the two ex-Soviet neighbors, as Moscow bitterly opposes Kiev's push to join NATO.

Ukraine's State Security Service said in its statement that Moscow Mayor Yuri Luzhkov's comments were undermining national interests and the country's territorial integrity. The agency also said it was looking into allegations that Luzhkov was involved in money laundering in Ukraine.

Russia's Foreign Ministry criticized the Ukrainian move as an "unfriendly step," but added that he was expressing the view shared by many Russians.

"Yuri Luzhkov only expressed an opinion which conforms with the viewpoint of the majority of Russians who have watched the breakup of the Soviet Union with pain," the ministry said in a statement.

Speaking Sunday in the Crimean port city of Sevastopol, Luzhkov was reported by the Interfax news agency as saying that the city belongs to Russia. A lease agreement with Ukraine allows Russia to use Sevastopol as the main base for its Black Sea fleet until 2017.

The Crimean peninsula has long been part of the Russian empire and then of Soviet Russia. In 1954, Soviet leader Nikita Khrushchev awarded the Crimea to Soviet Ukraine, where he lived and ruled for many years.

After the 1991 Soviet collapse, the Crimea became part of an independent Ukraine, causing a lot of discontent in Russia and among local residents, many of whom are ethnic Russians.

Luzhkov, who has long angered Ukraine with similar statements, reportedly claimed that Khrushchev never meant for Sevastopol, a key port where Russia's Navy had been based for centuries, to become part of Ukraine and suggested that the current arrangement should be reconsidered.

"This issue remains unresolved. We will resolve it within the framework of the truth, state positions and the rights that Russia has to its naval base — Sevastopol," he was quoted as saying by Interfax.

Russia's Foreign Ministry said that Luzhkov's statements reflected public dismay about the Soviet collapse.