PRAGUE, Czech Republic - US President George W. Bush accused Russia of backsliding on democratic reforms, but promised President Vladimir Putin he had nothing to fear from a US missile defense shield in Europe. "The Cold War is over," Bush said.

US President George W. Bush, left, and his wife Laura wave on their arrival in Prague, Czech Republic, Monday, June 4, 2007. Bush arrived in Czech Republic for a two-day working visit. [AP]

"Russia is not our enemy," Bush said Tuesday as relations between Washington and Moscow fell deeper into an icy chill with Putin's threat to retarget rockets at Europe.

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The accusations and finger pointing created a tense atmosphere for the annual summit of leaders of the world's most prosperous nations, beginning Wednesday in the Baltic Sea resort of Heiligendamm, Germany. Even before the missile shield dispute, the three-day meeting faced disagreements on issues ranging from global warming to aid for Africa.

US-Russia relations are arguably worse than they have been at any time since the collapse of the Soviet Union, with bitter differences across a range of issues. White House officials say they have been surprised by the depth of Putin's anger over the missile-defense plan.

Many ordinary Czechs worry the defense shield could make them terrorist targets. Czech President Vaclav Klaus raised those concerns during a meeting Tuesday with Bush in the medieval Prague Castle.

"It is very important that we win maximum support for this project of the Czech public who are very sensitive to those issues," Klaus said, standing alongside Prime Minister Mirek Topolanek. The Czech Republic would house the anti-missile radar system, while Poland would be the site for interceptor rockets.

"The people of the Czech Republic don't have to choose between being a friend to the United States, or a friend with Russia," Bush said. "You can be both."

With nerves jangled by the rising tensions, Bush felt it necessary to state what has been obvious for about two decades: "The Cold War is over. It ended," he said.

Bush said the sole purpose of the defense system is to defend against a rogue regime; US officials say he was talking about Iran and North Korea.

"As I've told President Putin, Russia is not our enemy," Bush said. "The enemy of a free society such as ours would be a radical, or extremists, or a rogue regime trying to blackmail the free world in order to promote its ideological objectives. And so my attitude on missile defense is, is that this is a purely - it's not my attitude, it's the truth - it's a purely defensive measure, aimed not at Russia, but at true threats."

Bush invited Putin to cooperate on missile defenses. "Why don't you participate with the United States," Bush said he would ask Putin when they meet. "Please send your generals over to see how such a system would work. Send your scientists."

The White House called attention to Putin's remarks in an NBC News interview seven years ago when he suggested that nations join in a common effort on missile defense against rogue nuclear powers.

"Such mechanisms are possible if we pool our efforts and direct them towards neutralizing the threats against the United States, Russia, our allies, or Europe, in general," Putin said then. "We have such proposals, and we intend to discuss them with President (Bill) Clinton."

A Kremlin official disputed Bush's assessment of Russia's democracy and questioned his assertions about missile defenses.

"Russia has been continuously moving to strengthen its democracy," said Dmitry Peskov, a deputy chief Kremlin spokesman who is in Germany for the summit. "Russia is a democratic state that shares European values."

"As for US assurances that missile defense sites in Eastern and Central Europe aren't aimed against Russia, we regrettably have failed to receive explanations of the feasibility of this anti-missile shield even though we have had continuous contacts on different levels," Peskov said. "Nor have we received an explanation whom these facilities will be targeting."

Bush arrived Tuesday evening in Germany unhappy about news from home - that former White House aide I. Lewis "Scooter" Libby was sentenced to 2 and a half years in prison for lying and obstructing the investigation into the leak of a CIA operative's identity.

Bush "felt terrible" for Libby and his family, spokeswoman Dana Perino said, but she refused to comment on whether Bush might some day pardon Libby.

Bush also prodded key allies in the war on terrorism - Egypt, Saudi Arabia, and Pakistan - "to move toward freedom."

The conference was hosted by Natan Sharansky, a former prisoner of the Soviet Union who has continued to champion freedom, and former Czech President Vaclav Havel.

Bush will spend three days in Heiligendamm, which has been encircled by razor wire-topped fence to hold back demonstrators. Along with Putin, Bush will be joined by the leaders of Germany, Britain, France, Italy, Japan and Canada. China's Hu Jintao will be among a group of other leaders who will meet on the summit sidelines with the G-8 leaders.