Russia is waiting for Bulgaria to explain its intention to join a new US missile defence system in Europe which came as an unwelcome surprise to Moscow, Russian Foreign Minister Sergei Lavrov said Sunday.

"We are waiting for explanations, we were promised those," Lavrov was quoted by the RIA Novosti news agency as telling reporters in Managua.

"We have already questioned our US partners in Washington through our ambassador as to the meaning of this, and why we have this Bulgarian surprise after the Romanian surprise," Lavrov said.

Bulgarian Prime Minister Boyko Borisov said Friday he favoured taking part in a new US missile defence system in Europe and a US ambassador indicated that talks have already started.

Neighbouring Romania has angered Russia by announcing it wants to host medium-range ballistic missile interceptors as part of a US missile defence system expected to be operational by 2015.

The US ambassador to Bulgaria James Warlick hinted in a public lecture that Washington was open for talks with Sofia on the issue.

Warlick played down Russian fears about the system by reiterating President Barack Obama's statement that America wanted "a strong, peaceful, and prosperous Russia."

"The United States will not ask the Bulgarian government or people to choose between Moscow and Washington," he added.

The United States in September shelved a plan -- that had also angered Russia -- to place missile defence facilities in the Czech Republic and Poland.

The Obama administration has since launched discussions with potential host countries on a revamped missile defence approach.