DEAUVILLE, France - Trying to move beyond years of inherited mistrust, President Obama and Russian President Dmitry A. Medvedev claimed progress Thursday but achieved no breakthrough on a U.S. missile-defense plan that Moscow is concerned could threaten its security.

The two leaders went out of their way to stress - four times over - that their relationship was good. But Medvedev also acknowledged: "It does not mean that we'll have common views and coinciding views on all the issues. It's impossible."

And a White House aide acknowledged that on the missile-defense question, for years the single most confrontational issue in the U.S.-Russian relationship, both sides still were trying to overcome "old thinking," and the Russians, in short, "don't believe us."

The two sides have long been in negotiations over U.S. intentions to station missile interceptors in Central and Eastern Europe. Russia believes the plan could threaten its own missile arsenal despite U.S. assurances to the contrary.

Medvedev expressed confidence the matter would be resolved, though not anytime soon - perhaps in the year 2020, he suggested.

Obama, for his part, said the two sides would keep working to find "an approach and configuration that is consistent with the security needs of both countries, that maintains the strategic balance and deals with potential threats that we both share."

Obama and Medvedev met for 90 minutes on the sidelines of a two-day summit of the Group of Eight industrial nations. The two leaders touched on a range of issues including the unrest sweeping the Mideast and North Africa, and Russia's efforts to gain entrance to the World Trade Organization.

In the meeting, Obama pointed to U.S.-Russian cooperation on a range of issues and said the two countries had successfully "reset" relations during his administration. But the missile dispute offered fresh evidence that the reshaping requires overcoming long traditions of mistrust.

"This is a very hard issue," said Michael McFaul, Obama's top adviser on Russia. "There's a lot of old thinking in both of our governments, frankly. This is a new challenge to think about how to do this cooperatively."

McFaul said that although U.S. officials had gone out of their way to demonstrate that the missiles would not be a threat to Russia, "they don't believe us."

While the two leaders appeared stern when they spoke with reporters at the end of their meeting, White House aides insisted they had a warm, free-flowing exchange and even joked together.

But Medvedev's pessimistic near-term read on the possibility of cooperation on missile defense was a disappointment for the Obama administration, which has been pushing for a breakthrough that could remove a major hurdle for new arms-control talks.

The administration had hoped that its move in 2009 to replace a Bush administration plan to install long-range missile interceptors in Eastern Europe had removed a major irritant in U.S.-Russian relations and that the issue could be effectively neutralized by limited cooperation on the issue.

Andrew Kuchins, a Russia expert at the Center for Strategic and International Studies, said that while the Russian "reset" effort was showing great momentum last year, "now it's kind of in a bit of a lull."