By Gabriela Baczynska

WARSAW (Reuters) - Poland fears planned defense cuts by the United States may stall the development of an anti-missile system on Polish soil and leave it more vulnerable to Russia, a senior government source said.

U.S. President Barack Obama's drive to cut nearly half a trillion dollars in defense spending over the next 10 years means Washington is reviewing already announced programs to reflect a stronger focus on Asia.

This review covers a planned U.S. missile shield endorsed by NATO. Missile interceptors are planned to be deployed in Poland from 2018 but the plan is vehemently opposed by its neighbor Russia, which regards the shield as a threat to its security.

"From Poland's point of view, the worst-case scenario is that Russia, sensitive to this issue, as a retaliation places various elements in Kaliningrad while the missile shield, now reviewed by Americans, never comes true," the source said under condition of anonymity because of the sensitivity of the matter.

Moscow worries the missile shield would undermine its nuclear deterrent capability. It has said it would take military counter-measures if needed and could deploy Iskander missiles in its Kaliningrad enclave, a region separated from the rest of the country and sitting between NATO members Poland and Lithuania.

In November President Dmitry Medvedev opened an early-warning radar facility there.

NATO dismisses Moscow's argument and says the shield is aimed at protecting its European allies from potential attacks by the so-called rogue states like Iran. But talks between Russia and the 28-nation alliance are stalled.

Poland, one of the most pro-American countries in Europe and once a member of the Soviet bloc, hopes enhanced military cooperation with the U.S. would upgrade its defense systems.

Washington already rotates a Patriot missile battery through Poland and last year sealed a deal with Warsaw on stationing U.S. air force personnel on Polish soil.

But this too may come into question, analysts say, under the new U.S. defense strategy.

(Editing by Robert Woodward)