Story highlights An interim ballistic missile defense system is aboard U.S. warships, an official says

The ships are in the Mediterranean and can be put under NATO control, an official says

NATO says it is inviting Russia to "cooperate on missile defense"

Russia believes that the system will be used as a deterrent to Moscow's systems

NATO's chief says the alliance now has interim ballistic missile defense capability in Europe, a move that is likely to further heighten tensions with Russia over its objection to a missile defense shield.

"It is the first step towards our long-term goal of providing full coverage and protection for all NATO European populations, territory and forces," Secretary-General Anders Fogh Rasmussen told reporters Sunday, the first day of a two-day NATO summit in Chicago.

Among the so-called interim capabilities are missile interceptors loaded on a U.S. ship in the Mediterranean, the first of four anticipated warships with the defense system, Rasmussen said. A defense radar is also operational in Turkey, he said.

NATO has asked Russia to participate in the system but negotiations have been deadlocked over Russia's demand for a legally binding treaty guaranteeing the shield would not be used as a deterrent to Moscow's own systems.

The Obama administration and its European allies have been trying to ease Russia's fears over the project by insisting that the system is directed toward countering the missile threat from the Middle East from which Russia also needs protection.

"We have invited Russia to cooperate on missile defense and this invitation still stands," Rasmussen said.

"We will continue our dialogue with Russia and I hope that at a certain stage Russia will realize that it is in our common interest to cooperate on missile defense."

Late last year, then-Russian President Dmitry Medvedev threatened to withdraw from the START treaty on nuclear weapons reductions and deploy ballistic missiles in its enclave of Kaliningrad on its border with Europe, if NATO moved ahead with the plans for missile defense.

Tensions further increased in December when Russia's ambassador to NATO, suggested Moscow would close transit routes that send vital supplies to troops in Afghanistan.

The issue hit a boiling point earlier this month when a Russian general, at the conclusion of the International Missile Defense Conference in Moscow, raised the possibility of a pre-emptive strikes against the defense shield's launch sites if a deal could not be reached.

The deadlock over the missile defense shield has also proved problematic for leaders from both the United States and Russia.

Since NATO approved the U.S.-designed system at a summit in Lisbon in 2010, Poland, Romania, Spain and Turkey have agreed to deploy parts of it.

The interim system, Rasmussen said, will link together the allies missile defense systems -- satellites, ships, radars and interceptors -- under NATO control from a U.S. base in Ramstein, Germany.

The full missile defense shield is scheduled to be operational by 2020.

U.S. Ambassador to NATO Ivo Daalder confirmed the ships were in the Mediterranean and "able to operate under NATO command when necessary in a crisis."

While neither Rasmussen nor U.S. officials would identify which NATO countries were offered protection by the interim defense shield, it is widely believed to portion of Southern Europe that might be susceptible to a ballistic missile strike from Iran or other Middle East countries.

Also Sunday, NATO leaders inked a deal to acquire five unarmed drones as part of "smart defense," a term used to describe efforts to do more with less at a time when many nations' defense budgets are being slashed, Rasmussen said. More than a dozen countries will help to buy the drones.