WARSAW, May 7 (Xinhua) -- Four expert teams will assess threats connected with the installation in Poland of a U.S. anti-missile shield and prospects of U.S. military aid for Poland, said participants in Wednesday's round of talks between Poland and the U.S. in Warsaw.

    Polish shield negotiator Witold Waszczykowski said the four teams will "provide knowledge on what Poland needs and who can pay for it", according to Polish news agency PAP.

    Stephen Mull, head of the U.S. delegation to the talks, said the teams would file a report on their findings by late July.

    Mull added that U.S. President George W. Bush has asked Congress for an extra 20 million U.S. dollars in military aid for Poland.

    Polish Defense Minister Bogdan Klich said Wednesday that U.S. military aid for Poland was the key issue in the talks.

    Asked about rumors that the United Stats was considering another site for the shield, Klich said it was still too early in the talks to say.

    "The talks are still underway and it's hard to say what their outcome will be. I have hopes for a good understanding," Klich said, adding that for the Polish side it was "not important when the talks will close but what effects they will bring."

    Talks on the shield between the United States, Poland and the Czech Republic opened at the start of last year. The U.S. plans to install in Poland a base of 10 interceptor rockets and a tracking radar in the Czech Republic, to shield the United States and its allies from attack by what it calls "rogue" states, particularly Iran.

    In return for its agreement to install the shield Poland wants the United States to participate in the modernization of the Polish army, mainly of its air defense.

    Russia has objected the anti-missile shield plan, saying it will threat Russian national security, and has warned that Moscow will target its missiles at the system if it is deployed in Poland and the Czech Republic.