Referring to negative comments from Moscow, following his visit there earlier this week, US Defense Secretary Gates said there were differing opinions within the Kremlin and the Russian military.

Gates, who was speaking after a meeting in Berlin with German Defense Minister Franz Josef Jung and Foreign Minister Frank-Walter Steinmeier, said he would return to Russia soon to continue discussions.

He said the US had offered to work closely with Moscow and negotiate limits that would ensure the shield would not pose a threat to Russia's nuclear deterence.

"We will continue to consult with Russia on missile defence as well as with our allies," Gates said. "We've made some very far-reaching proposals."

"There is some debate in Moscow about this dialogue," he added.

Attempts to alleviate Russia's concerns

However, addressing a press conference in Berlin with Gates, Jung said the aim was a "total concept" on missile defense and that the Russian objections were "unjustified."

The missile shield was intended as a "defensive function for the population," Jung said, adding that it was aimed at countering a threat that could be 10 years away.

Jung also said earlier in an interview with the daily Sächsische Zeitung published Wednesday that he was confident the United States could alleviate Russia's concerns.

Russia can be convinced of the necessity of the missile defense system "by explaining to them that there is a shared interest in security against missiles -- for example from rogue states," Jung said.

German Chancellor Angela Merkel has urged Washington to engage Russia on its concerns about US plans to deploy 10 interceptor missiles in Poland and a radar in the Czech Republic by 2012.

The four-billion-dollar project is aimed at countering a potential missile threat from Iran, not Russia.

Scepticism about plan

But the plan has antagonized Moscow, which sees it as a further military encroachment in its former sphere of influence, and has aroused misgivings among some European allies.

On Tuesday in Warsaw, Gates said he did not believe that Russia was a military threat to Poland "either now or (in the event that) we should install a missile defense."

"The world changes in dramatic ways, and what we are talking about here is indivisible security for the United States and for our NATO allies," he said.

"We would like to extend that umbrella to Russia, and partner with Russia and have Russia be with us in this program," he said.

Poland's willingness

Poland's conservative government, led by the president's twin brother, Prime Minister Jaroslaw Kaczynski, has signaled its willingness to host the base, but formal negotiations between Washington and Warsaw have not yet begun.

Gates, who started the trip Monday in Moscow, has invited the Russians to inspect a US missile defense interceptor site in Fort Greely, Alaska and a radar in California to clear up what he said was Russian misunderstanding about the capability of the system being proposed for Europe.

"In terms of assurances that the system would not be changed years from now in a way that might be more threatening to the Russian deterrent, it seems to me that it is a matter that can be negotiated," he said.

Gates provided no details and it was unclear whether the scope of such negotiations would involve the broader US missile defense system, or only the proposed European component.