MOSCOW - Defense Secretary Robert M. Gates failed yesterday to coax Russia from its opposition to U.S. missile-defense bases in Europe, but he suggested Moscow might be more flexible than it seemed.

Gates, on his first visit to the Russian capital since he came here as CIA director in 1992, made the opening bid in a series of high-level Bush administration moves aimed at softening Russia's view on missile defense. Secretary of State Condoleezza Rice will follow up this week at talks in Norway, and Gates said Russian and U.S. technical experts would confer in the weeks ahead.

Outwardly, Gates seemed to make little or no progress. But after a full day of meetings, including a session with President Vladimir V. Putin, he sounded a modestly optimistic note and said talks would continue.

"I felt like we made some headway," he told a small group of American and Russian reporters at his Moscow hotel.

"I think there are some misunderstandings" on the Russians' part, Gates said, about some of the technical aspects of the missile-defense system that the Pentagon wants to place in Eastern Europe. The intent is to give a degree of protection to Europe - including parts of Russia - from missiles fired from the Middle East. Washington is engaged in talks with Poland about building a missile-defense base there to house 10 interceptor rockets and with the Czech Republic about hosting missile-tracking radar.

President Bush spoke with Putin yesterday by telephone about missile defense and other issues, the White House said. The Kremlin said the call was initiated by the White House and included international political problems, cooperation on humanitarian issues and a planned meeting between the two leaders in June in Germany during the summit of major industrialized nations.

In Moscow, the Russians also raised with Gates their concern about a U.S. Navy plan to convert nuclear warheads of some of its submarine-launched Trident missiles to conventional warheads. The Russians' worry that a converted Trident missile could be mistaken for a nuclear launch, risking the possibility of a retaliatory nuclear strike. Gates said the warhead-conversion plan remained on the Pentagon's agenda.

Last week, U.S. officials gave the Russians a new package of suggested ways of cooperating on missile defense, a move intended to offset the Russians' concern about having the U.S. military so close to its border.

Gates said yesterday that these proposals included working together to experiment with new concepts and technologies as well as research and development, and possibly co-locating U.S. and Russian radar.