Russia should start preparing to withdraw its Black Sea Fleet from bases in the Crimea now, because the process could take up to six years, a Ukrainian diplomat said on Friday, RIA Novosti reported.

Under bilateral agreements, Russia`s Black Sea Fleet has the use of the Crimean Sevastopol base and other naval facilities until 2017. Ukrainian President Viktor Yushchenko announced in the summer that Ukraine would not extend the lease beyond that date.

"According to our estimates, the pullout and preparation of alternative facilities [for the Black Sea Fleet] may take a while. We expect this process to last five or six years," said Leonid Osavolyuk, director of the First Territorial Department at the Ukrainian Foreign Ministry.

"The Russian leadership must therefore start getting ready to withdraw its fleet as soon as possible," he told a news briefing at the ministry.

Osavolyuk said on Friday that Russian combat ships frequently transport undeclared cargo and refuse to submit customs declarations while crossing Ukrainian territorial waters, in violation of a decree issued by the Ukrainian president.

Although the agreement foresees a possible extension of the base lease, and Moscow has repeatedly said it wants negotiations on the issue, Ukraine reiterated its position on Thursday that it would not permit an extension of Russia`s naval presence in the country after 2017.

Tensions between Russia and Ukraine heightened after several Black Sea Fleet warships dropped anchor off the Georgian coast during and after August`s armed conflict with Tbilisi over breakaway South Ossetia.

President Yushchenko has called for the Russian Navy`s early withdrawal from the Sevastopol base, as well as tougher deployment requirements and higher fees, demands that have not been backed by his former coalition ally, Prime Minister Yulia Tymoshenko.

Russia condemned Yushchenko`s actions as a violation of the bilateral agreements on the presence of the Black Sea Fleet in Ukraine and said its fleet would continue carrying out its activities as usual.

Russia`s naval base in the Crimea currently has 50 warships and patrol boats, along with around 80 aircraft, and employs coastal defense troops.

The next round of Russian-Ukrainian talks on the presence of the Black Sea Fleet in Ukraine has been tentatively scheduled for November.

RIA Novosti