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.