WASHINGTON - President Bush said yesterday he was stepping up a dialogue with Russia over a planned U.S. missile-defense system in Europe in hopes of convincing Moscow it's only "a friendly force."

The issue came up on the first day of a U.S.-European Union summit at the White House. Bush said that at German Chancellor Angela Merkel's urging, he had begun trying to better explain his plans to President Vladimir V. Putin.

"Our intention of course is to have a defense system that prevents rogue regimes from holding Western Europe and/or America hostage," Bush said. "Evidently, the Russians see it differently."

Bush said he personally requested of Putin that he give Defense Secretary Robert M. Gates an audience on a recent trip to Moscow so that Gates could discuss the plan more fully.

"We have started a dialogue, as a result of Secretary Gates' visit, that hopefully will make explicit our intentions, and hopefully will present an opportunity to share with the Russians so that they don't see us as an antagonist force but see us as a friendly force," Bush said.

Under the plan, 10 missile interceptors would be placed in Poland and an early-warning radar system would be built in the Czech Republic. The United States is seeking permission from both countries for basing rights. Last week, Putin repeated opposition to the U.S. plan and threatened to pull out of a key post-Cold War treaty that set limits on the deployment of conventional military forces in Europe as a result.

Ahead of the talks with Bush, Merkel, whose country holds the EU's rotating presidency, proposed that Russia be invited to participate in a common-threat analysis to clarify the need for the defense system. She said that talks should take place in the NATO-Russia council.

Although she said she did not expect great progress on the impasse with Russia at the summit, she said she would press her concerns.

"I want to make clear again that things need to be discussed jointly with Russia," she said.