WARSAW, Poland — A U.S. missile defense system aimed at protecting Europe from ballistic missile threats is moving into higher gear this week, with a site in Romania becoming operational on Thursday and officials breaking ground at another site in Poland a day later.

The system has been under development for years and is, NATO and U.S. officials say, aimed against potential long-range threats from the Middle East, mainly with Iran in mind. Yet Russia is adamantly opposed to having the advanced military system on its doorstep and the development is certain to further exacerbate tensions between Russia and the West that are more strained that at any time since the Cold War.

The United States and NATO say the missile shield — which is able to track and shoot down incoming missiles — is purely defensive and is, in any case, powerless against Russia’s large stockpile of intercontinental ballistic missiles.

While the Kremlin doesn’t view the NATO missile defense system as a threat to its nuclear forces in its current limited shape, it fears that the U.S.-led missile shield may eventually erode the deterrent potential of Russian nuclear forces when it grows more powerful in the future.

Russian officials have shrugged off the claim that the planned missile shield is intended to fend off missile threats from Iran, and President Vladimir Putin has pointed at the determination of the U.S. and NATO to pursue the project even after a nuclear deal with Iran as a proof that it’s aimed against Russia.

Western officials deny that.

“Ballistic missile proliferation is a growing threat,” said NATO deputy spokeswoman Carmen Romero. “More and more countries are trying to develop or acquire ballistic missiles. Moreover, missile technology is becoming more sophisticated, lethal and accurate, and increasing in range.”

“For us to discount or ignore that very real missile threat would be irresponsible,” Romero said.