Russia will seek an explanation from Bulgaria about Sofia's plans to join a US missile shield, Russia's foreign ministry said Thursday.

Russian Deputy Foreign Minister Vladimir Titov met Bulgaria's ambassador to Moscow, Plamen Grozdanov, and told him Moscow would send Sofia an official query about the issue, the ministry said in a statement.

"In connection with the reports of consultations between Bulgaria and the United States on hosting elements of US missile defence ... the ambassador was told that Russia will send an official request for an explanation about the aims and existence of such talks," it said.

Last week, Bulgarian Prime Minister Boyko Borisov said he favoured taking part in a US missile shield, and the US ambassador to Bulgaria indicated that talks between Sofia and Washington had already started.

The comments drew an angry reaction from Russia, which has long been opposed to US plans to station missile defences in the former Communist countries of eastern Europe.

Bulgaria's neighbour Romania also upset Moscow this month by saying it would hold talks with the administration of President Barack Obama on hosting US missile interceptors.

Washington says the missile shield is designed to protect against short- and medium-range missiles from Iran and is not directed against Russia.