Russia was receptive but has stopped short of accepting a historic Nato invitation to join a missile shield protecting Europe against Iranian attack, the alliance's chief announced.
Russian president Dmitry Medvedev agreed to involve technicians in development plans, but did not make a commitment for his nation to be linked to it if it becomes operational, Nato chief Anders Fogh Rasmussen announced.
"We could cooperate one day in shooting down missiles," Mr Rasmussen said.
US president Barack Obama praised Russia's decision, saying it "turns a source of past threats into a source of potential cooperation against a shared threat".
Mr 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 programme the West says is aimed at producing a bomb.
Two key unanswered questions about the missile shield - will it work and can the Europeans afford it? - were put aside for the present in the interest of celebrating the agreement as a boost for Nato solidarity.
"It offers a role for all of our allies," Mr Obama told reporters. "It 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 neighbour was singled out.
Turkish President Abdullah Gul said that Nato met his nation's demands and that the agreement "was within the framework of what we wished. We are pleased about this".
Under the arrangement, a limited system of US 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 defences. That would create a broad system that protects every Nato country against medium-range missile attack.
