U.S. President George W. Bush risked escalating a rumbling row with Russia over his country's plans for a Europe-based missile defense system on Tuesday by warning that democratic reforms in Russia had been "derailed."

The comments came in a speech to a conference organized by former dissidents in the Czech capital, Prague, The Associated Press reported.

"In Russia, reforms that once promised to empower citizens have been derailed, with troubling implications for democratic development," said Bush, who is due to meet Putin on Wednesday ahead of this week's G8 summit in Germany.

But in an earlier press conference, Bush said Moscow had nothing to fear from U.S. missile shield plans, which would see bases placed in European countries once within the Kremlin's sphere of influence including the Czech Republic and Poland.

"Russia's not our enemy," Bush said, adding that the defense shield was meant to target "rogue nations," not escalate a new Cold War standoff.

Bush urged Russian cooperation with U.S. efforts to build a weapons system that would help "safeguard free nations against a missile attack launched from a rogue regime," inviting Putin to send military personnel and scientists to learn about the system.

"My message will be, Vladimir -- I call him Vladimir -- that you shouldn't fear a missile defense system," Bush said.

"As a matter of fact, why don't you cooperate with us ... Please send your generals over to see how such a system would work. Send your scientists. Let us have the ability to discuss this issue in an open forum."

"We'll be completely transparent," he added.

While the U.S. relationship with Russia is complex, Bush said the countries could work together to deal with mutual threats.

But top Russian officials have sharply objected to the deployment of missile-defense technology in Europe.

Putin has said the deployment threatens Russian territory and will "turn Europe into a powder keg and stuff it with new types of weapons."

The Kremlin leader raised the stakes over the weekend, warning that he may aim nuclear weapons at European targets unless Washington abandons its plans.

"If this does not happen, then we will withdraw any responsibility for our retaliatory measures because it wasn't us who initiated a new round of arms race development in Europe," Putin said.