The two leaders are set to meet at the NATO summit in Lisbon Saturday.
"One of the most important international topics in the near future will be the NATO summit in Lisbon," Medvedev told RIA Novosti after meeting with Obama privately at the Asia-Pacific Economic Cooperation summit in Japan.
Russia has opposed U.S. plans to deploy a missile shield in Eastern Europe. The United States says it would defend NATO nations against possible missiles from Iran, but Russia says there is no such threat.
NATO recently suggested that Russia join a missile defense project for Europe, but Moscow wants a serious assessment of missile risks first.
Some Russian and U.S. experts fear the missile defense controversy could block Senate ratification of the new nuclear arms reduction treaty.