In a joint appearance with President Bush on Monday, the two leaders showed signs of rapport and said they agreed on how to proceed with the missile defense talks. Tusk later cited personal trust in Mr. Bush as a sign that the negotiations were on track.
"The words of President Bush were very convincing," Tusk told The Associated Press through an interpreter minutes after leaving the White House. "This is a politician who is controversial for some but, in my opinion, is very trustworthy. I believe that is extremely important in the world of politics."
The talks had been complicated by Poland's demand for help in upgrading its military in exchange for allowing the interceptors. U.S. negotiators wanted to deal with the Polish demands separately and leave promises vague.
But despite Mr. Bush's reassurance, Tusk made clear that he will be waiting to see a more concrete offer from the United States. Mr. Bush promised he would offer specifics "before my watch is over" in January.
"Poland is a little nervous about Russia and they are looking at what happens in 2009," said Steven Pifer, a former U.S. diplomat who is now an analyst at the Center for Strategic and International Studies. "Having a good meeting was important, but I suspect that as they do the details, Poland is going to want to get something locked in concrete."
The Bush administration has been seeking to begin construction of its European missile shield while it is still in office and to complete it by 2012. The plan also includes installing a radar in the Czech Republic. But because the negotiations with Poland are lagging and any deals would have to be approved by the Polish and Czech parliaments, it may be difficult for the U.S. administration to meet its timetable.
Polish supporters of the plans are concerned that a new U.S. administration could kill the project. Among the major candidates to succeed Mr. Bush, Republican John McCain is a strong supporter of the U.S. missile defense program, while Democrats Barack Obama and Hillary Rodham Clinton have been less vocal on the issue.
The U.S. missile defense plans have become one of the thorniest issues in U.S.-Russian relations. Russia opposes the U.S. plan to build part of its global missile defense system so close to Russian borders, arguing that it would undermine the Russian deterrent. The Polish government argues that the security backing is necessary because Russia has threatened to target Poland with nuclear missiles if it should allow the interceptors.
Tusk said that Mr. Bush had assured him that the United States would continue to try to persuade Russia that the missile shield was not a threat. The United States says that it is aimed at countering a threat from countries such as Iran and North Korea.
"The Polish point of view has been accepted and I treat that as - maybe not a breakthrough because I don't want to use big words - but a very clear explanation of our mutual intentions. This will allow our negotiators to continue their work," Tusk said. "No one will make haste on this. We want to negotiate a deal that will be good for Poland but also for the U.S. It does not mean it has to be immediately."
Neither leader talked specifics. Mr. Bush said "obviously there's a lot of work to do" and that experts are working through the details to make sure that "the people of Poland are comfortable with the idea."
Polish officials have said they are looking for help to acquire air defenses against short- to medium-range missiles. Negotiators have asked for Patriot 3 or THAAD missiles and have identified 17 areas of the Polish military that the United States could help modernize. Interceptors for the planned U.S. shield are for protection against long-range missiles.
Asked if he thought a deal would be reached while Mr. Bush was in office, Tusk demurred.
"I am not an expert on U.S. politics nor am I a prophet," he told the AP. "I am not going to tell fortunes."