Russia's parliament on Wednesday recommended that the Kremlin consider pulling out of a friendship treaty with Ukraine if the ex-Soviet state takes further steps to join NATO.
Parliament's resolution is not binding and the Kremlin has not expressed its position, but the move is likely to add to pressure on Ukraine days before its pro-Western leader meets Russia's new President Dmitry Medvedev.
Moscow is opposed to Ukraine joining NATO, saying that would threaten Russian security and jeopardise an arrangement under which Russia leases Ukraine's Black Sea port of Sevastopol as a base for its navy.
The pro-Kremlin parliament recommended the government withdraw from a 1997 friendship treaty if Ukraine is given a NATO Membership Plan, a roadmap to membership, or other steps are taken to speed up its accession to the alliance.
Scrapping the treaty could, in theory, open the way for Russia to mount a legal challenge to Ukraine's sovereignty over Sevastopol.
Kiev says it is part of its territory. The treaty recognises the port as within Ukraine's borders but Russian legal experts say without this document, the legal grounds for its status as part of Ukraine are shaky.
The Crimean peninsula, which includes Sevastopol, was part of the Russian republic of the Soviet Union until 1954, when Soviet leader Nikita Khrushchev signed it over to the Ukrainian republic.
Some in Russia say Khrushchev's decision was illegal.
The State Duma, or lower house of parliament, voted by 408-5 to adopt a resolution on Ukraine that included the friendship treaty recommendation.
The Kremlin has a majority in the State Duma. It often uses the chamber to issue tough resolutions on disputes with its neighbours, a ploy analysts say is designed to strengthen the Kremlin's negotiating position.
Ukrainian President Viktor Yushchenko is expected to meet Medvedev later this week at an informal summit of ex-Soviet states. The talks will be their first since Medvedev took over from Vladimir Putin as president last month.