Russian President Dmitry Medvedev says time is running out for the United States to come up with a plan to secure Russia's agreement to a U.S.-led NATO missile defense system in Europe.

In a speech Friday, Mr. Medvedev said it would be in the interest of both countries to quickly reach mutually acceptable agreements. He said Moscow remains unconvinced that the planned missile shield is intended as protection against a missile attack by countries such as Iran or North Korea.

Mr. Medvedev's comments come ahead of talks next week with U.S. President Barack Obama at the Nuclear Security Summit in Seoul, South Korea.

Earlier this week, the Russian leader told senior military officials that Russia must prepare itself to respond to U.S. missile defense plans despite ongoing dialogue between Moscow and Washington.

Russia has been objecting to the missile defense system in Europe since its conception, saying it threatens Russian security. NATO has said it wants to cooperate with Russia on the missile shield but has rejected Moscow's proposal to run it jointly. Without such an agreement, Russia wants a legally binding guarantee from the U.S. that any future missile defense is not aimed at Russia.

The missile shield will be discussed at the NATO summit in Chicago in May, after a Group of Eight summit at Camp David. By then, Vladimir Putin will be sworn in as Russia's next president. It is unclear if he will attend the NATO summit.