The U.S. State Department, however, did not rule out a stop for Rice in Poland should negotiations in the coming days produce positive results.

"She's going to be traveling in Europe and if there were a possibility of signing something, that would have been positive," State Department spokesman Sean McCormack said on Monday.

The U.S. shield is also expected to be the backbone of NATO's missile defense system. But the alliance has a problem as the U.S. system in Europe fails to satisfy the so-called principle of "indivisibility of security," which means all allies must enjoy the same degree of protection.

The U.S. system will leave parts of Bulgaria, Romania, Greece and almost all the territories of Turkey unprotected as these areas are too close to the potential source of threat.

As a reaction to the U.S. plan, Moscow has threatened to aim its nuclear warheads toward Central and Eastern Europe should the plan be allowed to go ahead.

Moscow is not buying Washington's explanation that 10 interceptor missiles in Poland would not undermine Russia's deterrence. Moscow argues that national security shall not be based on verbal promises as the number of interceptors may increase in the future.

(Xinhua News Agency July 8, 2008)