Poland and the United States are stepping up talks aimed at reaching a deal on a controversial U.S. plan to deploy parts of a missile defense shield on Polish soil.

U.S. diplomat Stephen Mull said Wednesday in Warsaw that the two sides will form working groups of experts to explore issues of concern. He said Washington would seek other deployment sites in Europe if the current talks fail.

Donald Tusk (file photo)

Bilateral talks have been going on for months, aimed at reaching agreement on the deployment of 10 missile interceptors in Poland. The U.S. plan calls for guidance radar in the Czech Republic.

Tuesday, Polish Prime Minister Donald Tusk said his government will block U.S. deployment plans, if Washington fails to satisfy Warsaw's demands for assistance in upgrading Poland's military.

U.S. officials say the system would defend against the missile threat from countries such as Iran. Russia firmly opposes the system, saying it is a threat to its security.

Some information for this report was provided by AFP, and Reuters.