Bogdan Klich's remarks, published Monday in a newspaper interview, underlined the shift in thinking on the issue under Poland's new government, which took power on Friday. The new prime minister, Donald Tusk, and his pro-business government have vowed to take a firmer stand in its relations with the United States.
Under former Prime Minister Jaroslaw Kaczynski, Warsaw began negotiations with Washington on its request to place 10 interceptor missiles in the country, frequently expressing strong support for the plan as a way to strengthen the trans-Atlantic alliance.
But in an interview with the Dziennik daily, Klich said that Poland must once again "weigh the benefits and costs of this project for Poland. And if that balance results unfavorably, we should draw a conclusion from those results."
There is some anxiety in Poland that Warsaw could further strain its already shaky ties with Moscow by agreeing to base the missile defense interceptors on its territory.
Russian officials have repeatedly warned that the U.S. plan could lead to a new arms race, with a senior general warning last week that Russia could send short-range missiles to Belarus - which borders Poland - as part of efforts to counter the planned missile defense sites.
Other Polish leaders, including new Foreign Minister Radek Sikorski, have warned against provoking Russia without first being sure that the U.S. Congress will indeed back up the plan with funds.