"The American shield will be deployed here only if it raises our security," Donald Tusk told a news conference given by the government.
He said that if Warsaw is to allow the plans to go ahead, the U.S. must assist in modernizing the country's armed forces.
Under the U.S. plans, 10 missile interceptors will be deployed in Poland and a radar site in the neighboring Czech Republic. Moscow is opposed to the plans, saying the shield would threaten Russia's national security.
Tusk said the U.S. is still a strategic partner of Poland, but warned against being hasty in talks on the issue. "Without damaging relations with the U.S., we would like to obtain a result that will demonstrably raise our security," he said.
Polish Foreign Minister Radoslaw Sikorski told the news conference that if the base is deployed in Poland, Warsaw is ready for Russia to inspect it. However, he said: "Don't confuse this with the direct presence of Russian servicemen at the U.S. missile defense base."
The Czech government approved the deployment of the missile defense system on its territory on Wednesday. The basic document still needs to be ratified by the country's parliament and signed by President Vaclav Klaus.
Prague and Washington are expected to sign two agreements on the deployment of a radar base in the Czech Republic. In addition to the missile defense treaty, the U.S. and the Czech Republic are to sign the SOFA treaty on the stationing of U.S. troops in the country.
On Thursday, the U.S. House of Representatives reduced funding for the project by 52%, approving a sum of $370.8 million. The Bush administration had originally requested $712 million in funding for the European component of the missile shield in 2009.
The U.S. Senate is due to consider the proposed missile defense bill in June.
