Systems to protect Europe from missile attack risk being ineffective and a threat to stability if they do not include Russia, President Dmitry Medvedev warned Saturday.

The Kremlin said Medvedev has written a letter to NATO heads of states to make clear Russia's position on missile defence amid continued tensions with the West over the longstanding dispute.

The letter appears to be a sign of growing frustration from Moscow that it is being sidelined by the West in discussions on the issue despite signs of progress late last year.

"A European missile defence system can only be genuinely effective and viable if Russia participates in an equal way," the Kremlin quoted Medvedev as saying.

Medvedev said it was necessary to be sure that the missile defence systems placed in Europe do not "disrupt strategic stability and will not be directed against either of the sides."

Russia earlier this month reacted with concern to an agreement between the United States and Romania to place US missile interceptors at a Soviet-built airbase in the EU member state.

At a summit of NATO leaders in Lisbon in November, Medvedev proposed that Europe be divided into sectors of military responsibility, including one overseen by Russia and one by NATO, to better protect the continent.

But so far the plan, which was hailed by Russian officials as a chance for a major breakthrough in Moscow-NATO ties, appears to have attracted little interest in the West.