The plan has sparked protests from Russia and Ukraine, and the Czech Republic has asked for it to be put on agenda of the Nato-Russia Council meeting in Seville, where frank talks were expected.

Last week, President Vladimir Putin said Russia would come up with a "highly effective" strategic response if the plans went through, adding that his country already had the missiles to overwhelm a US defence shield.

US Defence Secretary Robert Gates met here Thursday with Ivanov, and although they made no public comment, Russian concerns about the US plans were expected to be a subject of discussion.

"We do have differences. There's no question about that," Gates said, without elaborating, after Thursday's meeting. "But I think having conversations like this, having a frank discussion is clearly the best way to go."

Ahead of the Nato meeting, a senior US official insisted that the shield posed "no threat to Russia" and that the United States remained "open to future cooperation" on it.