Russia has serious questions over the aims of fresh US missile defence plans for Europe, the foreign ministry said Friday, in Moscow's strongest expression of concern yet over the new initiative.

"We still have serious questions regarding the true purpose of the US missile system," a ministry spokesman, Andrei Nesterenko, told reporters in televised remarks.

"Therefore we will continue to consistently oppose any dubious unilateral actions in relation to missile defence that can negatively affect international security."

His comments were Russia's strongest statement of concern after Romania said it would hold talks with Washington on hosting US missile interceptors and Bulgaria showed interest in taking part in a US missile shield.

Until now, the foreign ministry had only said it was concerned and was analysing the issue.

Nesterenko said: "We are again becoming witnesses to a hasty anti-missile arrangement for Europe when the fragile architecture of European security essentially becomes hostage to imaginary missile threats that are defined unilaterally."

Nesterenko voiced regret that Moscow was learning about the new US missile plans for Romania from media reports rather than directly from Washington and Bucharest, noting that it ran counter to Russia's idea of "equal partnership."

Last year Russia applauded US President Barack Obama's decision to shelve an initiative by his predecessor George W. Bush to place an anti-missile radar facility in the Czech Republic and interceptors in Poland.

But the apparent involvement of former Communist states Bulgaria and Romania in the Obama administration's replacement plan has irked Russian officials.

Earlier this month, Russian Defence Minister Anatoly Serdyukov warned that Moscow could revive plans to base missiles in its western Kaliningrad exclave, located on NATO's doorstep, if the threat warranted such a move.

However, the Kremlin indicated on Friday that a central element of Obama's efforts to improve US-Russian relations -- the push to replace a key nuclear disarmament treaty that expired in December -- was still on track.

President Dmitry Medvedev's top foreign policy aide, Sergei Prikhodko, said that Moscow believed a replacement treaty for the 1991 START pact could be agreed within the next two months.

"We still consider March-April to be a realistic timeframe," Prikhodko told reporters.