"We still agree that it is fundamental for us to maintain our alignment with the United States, which has been, is and will continue to be our strategic ally."
In the US, the state department said it was studying Tusk's remarks closely.
Sean McCormack, a state department spokesman, said: "Poland remains a close and important ally of the United States. We remain in negotiations with Poland and do not plan to comment publicly on the details."
Tusk, without disclosing full details, said Washington was proposing to put batteries of Patriot missiles on Polish soil for one year.
In the months-long negotiations, Tusk's government had sought billions of dollars worth of US investment to upgrade Polish air defences in return for hosting 10 two-stage missile interceptors.
"We are ready to accept proposals or corrections from the American side which would include our proposal to increase [our] security. We can do this in a day, a week, a month," Tusk said.
The Czech Republic has agreed to host a tracking radar under the project, though its parliament has still to ratify the deal.
Russia has condemned the missile defence plan as a threat to its own security and has said it will target missiles at Poland and the Czech Republic if the deployment goes ahead.
Tusk said: "If the threat related to the shield indeed increases, then we need elements such as Patriots on Polish territory, and not just for one year."
|Tusk said the missiles needed to be in place for over a year [GALLO/GETTY]
Political analysts said Tusk's rebuff to the US demonstrated a new Polish self-confidence on the global stage.
Warsaw is one of Washington's strongest allies in the region and has troops serving in Iraq and Afghanistan.
Pawel Swieboda, head of demosEuropa, a Warsaw think tank, said: "This is the first time that Poland has said 'no' to the US ... It certainly sends a signal to Washington that Poland's support should not be taken for granted in any circumstances.
"But it is also the case that the government greatly raised expectations and that these were never very realistic. Poland does need some compensation [for hosting the interceptors] but they went too far in demanding Patriots."
Lech Kaczynski, Poland's president, is an critic of the Tusk government and a strong supporter of the shield plan. He is now likely to step up his attacks on the government.
But analysts said the domestic political fallout would be limited as Polish voters are largely hostile to the project.
Washington has said it wants Poland to host the interceptor missiles but that it would look elsewhere if Warsaw declines to take part.
Lithuania has been suggested as an alternative site for the missiles.