The United States said Tuesday that it continues to believe it can strike a deal to set up a missile defense base in Poland, despite negotiations bogging down in a dispute over military aid for Poland.

Polish and U.S. envoys were meeting Tuesday and Wednesday on the U.S. proposal to place 10 missile defense interceptors in northern Poland as part of a global missile defense system.

The negotiations came as the Lithuanian prime minister visited Washington for talks with U.S. officials. Reports have suggested that the U.S. is eyeing Lithuania as an alternative missile site.

Asked about Lithuanian Prime Minister Gediminas Kirkilas’ visit, State Department spokesman Tom Casey told reporters Tuesday that the U.S. discusses missile defense with all NATO allies.

But, he said, “we are not, at this point, involved in any negotiations on alternative sites, because our goal is to conclude an agreement with Poland. If, for some reason, those arrangements don’t work out, then I’m sure we’d look elsewhere.”

The Polish Embassy in Washington declined to comment on the talks.

Poland’s prime minister, Donald Tusk, said in Warsaw Tuesday that an eventual deal on accepting a U.S. missile defense base must strengthen his country’s security.

Tusk dismissed suggestions that negotiations have stalled, distancing himself from remarks earlier in the day by his chief aide, Slawomir Nowak, who faulted Washington for a slowdown in talks. Nowak told state radio that “the Americans have toughened their position in talks with our negotiators. We see no progress.”

But Tusk disagreed.

“My government is not acting like a naive enthusiast, but like a hard negotiator with the awareness that we are negotiating with friends and our most important ally,” Tusk said. “But Poland’s security is a holy thing, and I will not allow for even the smallest mistake to be committed, and that’s why the negotiations aren’t as simple as they once seemed.”

Russia has sharply protested Washington’s plans, arguing that U.S. military installations so close to its borders threaten its security. Moscow has threatened to aim its own missiles at any eventual base in Poland and the Czech Republic with its own missiles.

Washington says the plan poses no threat to the Kremlin’s vast nuclear arsenal, and has stressed that the system aims to protect Europe and the United States from possible missile attacks from Iran.

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AP writer Monika Scislowska contributed to this report from Warsaw.