Mr Tusk said negotiations with Washington were ongoing
Poland's prime minister has rejected the latest terms offered by the US to host a missile defence shield.
Donald Tusk said any deal must increase Poland's own security, and he dismissed the latest offer as unsatisfactory.
"We are ready to co-operate but we are waiting for an answer from the American side," Mr Tusk told a news conference.
The US wants to place 10 missile interceptors in Poland as part of a defence shield against possible attack, a move condemned by Russia and China.
Russia has said the project would upset the military balance in Europe and warned it could have no choice but to point its own missiles at the installations.
The US is also seeking permission to build a radar station in the Czech Republic as part of the defence shield plans.
US Secretary of State Condoleezza Rice is expected to sign a deal with the Czech government next week in Prague.
Washington says the shield will help protect the country and its allies from so-called "rogue states" like Iran.
But Mr Tusk said that the presence of the missile shield in Poland would increase certain threats and risks for the country.
He did not, however, rule out the possibility of the issue being resolved.
"I wouldn't talk about the end, suspension or interruption," Mr Tusk said. "Negotiations, in my opinion, are continuing."
Poland has requested the additional security of a Patriot missile battery and has also sought billions of dollars worth of investment to improve its air defences.