Russia was receptive but stopped short of accepting a historic NATO invitation Saturday to join a missile shield protecting Europe against Iranian attack.

Russian President Dmitry Medvedev agreed to involve technicians in development plans, but did not make a commitment if it becomes operational and warned that Russia might decide against joining the U.S.-led effort if it doesn't feel it is being treated equally as a partner.

"Our participation has to be a full-fledged exchange of information, or we won't take part at all," he told reporters after the announcement by NATO chief Anders Fogh Rasmussen.

President Barack Obama won NATO support a day earlier to build the missile shield over Europe, an ambitious commitment to protect against Iran' increasingly sophisticated ballistic missiles and a nuclear program the West says is aimed at producing a bomb.

Obama praised Russia's decision Saturday, saying it "turns a source of past tensions into a source of potential cooperation against a shared threat."

Rasmussen was similarly upbeat: "We could cooperate one day in shooting down missiles."

Two key unanswered questions about the missile shield — will it work and can the Europeans afford it? — were put aside for the present by NATO members in the interest of celebrating the agreement as a boost for NATO solidarity.

Medvedev addressed those issues point blank, saying "it is quite evident that the Europeans themselves don't have a complete understanding how it will look, how much it will cost. But everybody understands the missile defense system needs to be comprehensive."

NATO says the cost of the system would be relatively cheap when spread across the entire 28-nation alliance — euro200 million euros, or about $260 million, over 10 years. But critics contend that's a big pricetag for Europe, suffering from a debt crisis that has led to higher unemployment while forcing governments to raise taxes, cut services and slash civil servant salaries amid austerity drives for many nations.

Obama said the missile system "responds to the threats of our times. It shows our determination to protect our citizens from the threat of ballistic missiles." He did not mention Iran by name, acceding to the wishes of NATO member Turkey, which had threatened to block the deal if its neighbor was singled out.

Turkish President Abdullah Gul said Saturday that NATO met his nation's demands and that the agreement "was within the framework of what we wished. We are pleased about this."

And France, which had had reservations that the missile shield plan might come across as a substitute from nuclear deterrence, said it too had signed on after its concerns were answered.

"France would have refused a unilateral project disconnected from reality, or costly — or if it had been for that matter hostile to Russia or had been a substitute for nuclear deterrence," French President Nicolas Sarkozy said.

He noted that no country was specifically mentioned as the object of the missile defense, but added: "France calls a cat a cat: the threat of the missiles today is Iran."

Under the arrangement, a limited system of U.S. anti-missile interceptors and radars already planned for Europe — to include interceptors in Romania and Poland and possibly radar in Turkey — would be linked to expanded European-owned missile defenses. That would create a broad system that protects every NATO country against medium-range missile attack.

Medvedev on Saturday joined a meeting of NATO's 28 leaders — a gesture that marked a sea change for a partnership created after World War II to defend Western Europe against the Soviet threat.

The allies opened their summit by agreeing on the first rewrite of NATO's basic mission — formally called its "strategic concept" — since 1999. They reaffirmed their bedrock commitment that an attack on one would be treated as an attack on all. In that context, the agreement to build a missile defense for all of Europe is meant to strengthen the alliance.

What remains in conflict, however, is the question of the future role of nuclear weapons in NATO's basic strategy. The document members agreed to on Friday says NATO will retain an "appropriate mix of nuclear and conventional capabilities" to deter a potential aggressor. Germany and some other NATO members want U.S. nuclear weapons withdrawn from Europe.

On the topic of a U.S.-Russia arms treaty, Obama was backed by Fogh Rasmussen, who told reporters that the treaty, called New START and signed in April by Obama and Medvedev, would improve security not only in Europe but beyond.

Ministers from six European countries — Denmark, Lithuania, Latvia, Hungary, Norway and Bulgaria — on Saturday urged U.S. lawmakers to ratify the stalled nuclear treaty, saying failure to do so would be a setback for European security.

The pact would reduce the limits on strategic warheads held by the U.S. and Russia and would establish an inspection system. It would be a major setback for Obama if he's unable to get it ratified by the Senate after inking it with Russia's president earlier this year.

NATO and Moscow signed an agreement to expand the alliance's supply routes to Afghanistan through Russia and were expected to set up a new training program in Russia for counter-narcotics agents from Afghanistan and other Central Asian countries; and agree on a program to provide training to Afghan helicopter crews.

Spanish Prime Minister Jose Luis Rodriguez Zapatero hailed the expanded security cooperation, saying "it opens an unprecedented field of cooperation and expectations between NATO and the political and military power that is Russia."


AP writers Robert Burns, Slobodan Lekic and Julie Pace in Lisbon, Jamey Keaten in Paris and Harold Heckle in Madrid contributed to this report.