Russia is demanding explanations from Bulgaria about its plan to host elements of the U.S. missile defense system, the Russian Foreign Ministry said on Thursday.

"As regards news about consultations Bulgaria is holding with the United States on the deployment of the U.S. missile defense system on its territory, Russia is sending an official letter, requesting explanations about the goal and essence of the talks," the foreign ministry said in a statement.

The issue was raised by Russian Deputy Foreign Minister Vladimir Titov while meeting with Bulgarian Ambassador to Russia Plamen Grozdanov, the statement said.

Romania has angered Russia earlier this month by announcing it wants to take part in the revamped U.S. missile defense system. News that Sofia was also ready to be part of the U.S. missile shield further raised concerns in Moscow.

In September, U.S. President Barack Obama scrapped Bush-era missile defense shield program, which Moscow strongly opposed, while initiating a "phased, adaptive approach" of the plan in Eastern Europe.

Russia initially welcomed Obama's announcement by officially declaring to ground plans installing short-range Iskander missiles in its western Kaliningrad enclave, but has urged Washington to further expound the new approach.