Russian President Dmitry Medvedev told his Ukrainian counterpart on Friday that Ukraine could be in breach of a friendship treaty between the two countries if it joins NATO, Russia`s foreign minister said, according to Reuters.

At a meeting with Ukrainian President Viktor Yushchenko in St Petersburg, Medvedev also urged Kyiv not to take any unilateral steps to expel the Russian navy from the base it leases on Ukraine`s Black Sea coast.

Ukraine`s Western-leading leaders have secured a commitment from NATO to admit it to the alliance eventually, a move that irked Russia, and they say they want to review the presence of the Russian Black Sea fleet in the Ukrainian port of Sevastopol.

Speaking after Medvedev and Yushchenko met, Russian Foreign Minister Sergei Lavrov said both the NATO and Sevastopol issues were governed by a 1997 friendship treaty signed by the two countries.

"The treaty between Russia and Ukraine .. contains the obligation on the two parties not to do anything which would create threats or risks for the security of the other party," Lavrov told a conference call with reporters.

"We do believe NATO expansion, which would include Ukraine, would create a risk for Russian security," he added.

"This was reiterated by President Medvedev, that we do not believe NATO membership for Ukraine would serve ... the interests of the two countries."

On the issue of Sevastopol, the Russian foreign minister said Medvedev had "called upon our Ukrainian colleagues to ... resolve all issues related to the functioning of the Black Sea fleet ... avoiding any unilateral acts."