BERLIN — The Obama administration has developed possible alternative plans for a missile defense shield that could drop hotly disputed sites in Poland and the Czech Republic, a move that would please Russia and Germany but sour relations with American allies in Eastern Europe.

Administration officials said they hoped to complete their months-long review of the planned antimissile system as early as next month, possibly in time for President Obama to present ideas to President Dmitri A. Medvedev of Russia at a meeting in New York during the annual opening at the General Assembly of the United Nations.