For six years, NATO has been working on building up a missile defence system in Europe, spurred on by the United States, which says it fears an attack from Iran.
But the project has left the military alliance at odds with Russia, which believes that the project jeopardizes its national security.
Question: Why does NATO say it needs a missile defence shield in Europe?
Answer: The military alliance believes that it is at risk of missile attacks from "outside the Euro-Atlantic area." The United States has specifically pointed the finger at Iran.
Washington has been working on establishing a missile defence system in Europe for years. In 2010, NATO leaders called for the creation of an alliance-wide anti-missile system.
NATO has maintained that the shield is to be used only for defensive purposes and is meant to act simply as a deterrent.
Q: Why doesn't Russia believe NATO's claim?
A: The two sides are geopolitical foes, and Russia views NATO's eastward expansion as a direct threat.
NATO's first secretary general, British General Hastings Ismay, reportedly said during the alliance's inception in 1949 that its goal was to "keep the Russians out, the Americans in, and the Germans down."
Since 1999, a dozen countries that were part of the former Eastern Bloc have joined NATO.
Q: What does Russia say the new system could be used for?
A: Russia has argued for almost a decade that what NATO calls a missile shield actually has the capability of launching an attack on Russia.
Mikhail Ulyanov, head of the Russian Foreign Ministry's department for nonproliferation of weapons, said on Wednesday that the NATO system could be able to fire long-range cruise missiles.
Russia also insists that the system, which NATO says would use interceptor missiles to shoot down incoming projectiles, is too close to its borders and too far from the Middle East to justify the claim that the purpose is to defend against Iran.
Q: What is NATO's response?
A: The military alliance has stuck by its assertion that it would be impossible for the system to be used against Russia.
NATO Secretary General Jens Stoltenberg said Thursday that "geography and physics" make it impossible for Russian intercontinental missiles to be shot down, arguing that the shield has too few interceptors and that they are "too far south or too close to Russia."
But the alliance has refused to grant Moscow the legally binding guarantees it demanded. NATO countries have argued that it is unreasonable for a non-alliance nation to want control over one of their defence systems.
Q: How has Russia reacted?
A: Russia has cited the missile system as a reason for it to heavily increase its military presence in its western-most region, Kaliningrad, an exclave between NATO member states Poland and Lithuania.
This week, the head of Russia's strategic missile forces, General Sergei Karakayev, warned that his country was developing a new intercontinental ballistic missile capable of penetrating the NATO shield.
Q: Is there any chance that NATO and Russia will manage to overcome their differences?
A: The relationship between the two sides is already fraught over the Russian annexation of Ukraine's Crimea peninsula and Moscow's support for separatists in eastern Ukraine.
NATO ambassadors and their Russian counterparts met last month for the first time in almost two years, but Stoltenberg said afterwards that the two sides still have "profound" disagreements.
He nevertheless vowed on Thursday that NATO would "continue to engage in dialogue with Russia when and where we can."
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