NATO chief Anders Fogh Rasmussen says the alliance has conducted a successful test of its missile defense system for Europe.

In an opinion piece in The Wall Street Journal, Rasmussen said NATO plans to declare an “interim capability” for the anti-ballistic missile system at a conference later this month. He said the system will still be “limited,” but that the development will mark the first “real step” toward covering all NATO countries in Europe.

Rasmussen stressed the threat of ballistic missiles, saying more than 30 countries have such technology. He said that while NATO prefers to resolve problems diplomatically, the alliance must be able to respond to an “aggressor” that rejects those efforts.

Russia has objected to the missile defense system in Europe since its conception, saying it threatens Russian security. The country's defense minister said earlier this month that Russia, NATO and the United States have not found a “mutually acceptable solution.”