DEVESELU, Romania — A U.S missile defense site in Romania aimed at protecting Europe from ballistic missile threats becomes operational Thursday, angering Russia, which opposes having the advanced military system in its former area of influence.

NATO Secretary General Jens Stoltenberg is due to speak at a ceremony later attended by U.S., NATO and Romanian officials to mark the start of operations at a base established by the Soviet Union, 110 miles southwest of Bucharest.

President Klaus Iohannis said Romania wanted NATO to have a "permanent naval presence" in the Black Sea that respected international conventions, and called for increased security for NATO members in the south and east, which border Russia and the Middle East.

"It is important that a credible and predictable presence can be assured of the allied forces on the eastern flank, to balance the northern dimension with the southern and eastern flank," Iohannis said after meeting Stoltenberg early Thursday in the Romanian capital Bucharest.

U.S. officials say the Romanian missile shield, which cost $800 million, is intended to fend off missile threats from Iran and is not aimed at Russia.

However, Adm. Vladimir Komoyedov, chairman of State Duma's defense committee, called the missile defense site a threat to Russia, which will bolster its missile defense systems earlier than planned.

"This is a direct threat to us," Komoyedov, the former commander of the Russian Black Sea Fleet, told the Interfax news agency. "They are moving to the firing line. This is not just 100; it's 200, 300, 1,000 percent aimed against us.

"This is not about Iran, but about Russia with its nuclear capabilities."

On Friday, Polish and U.S. officials will take shovels in hand to break ground at a planned site in the Polish village of Redzikowo, near the Baltic Sea.

Nataliya Vasilyeva in Moscow contributed to this report.