Rice had been hoping to clinch a deal before leaving for Prague later Monday to sign a related agreement for a missile-tracking radar in the Czech Republic, her spokesman Sean McCormack told reporters afterward.

"We didn't conclude them (the negotiations) in time for the beginning of the secretary's travel. That doesn't mean we're not going to keep working on them," McCormack said after the top diplomats held quickly-convened talks.

McCormack acknowledged that the United States had hoped to reach an accord with Poland now that would allow her to end her tour of Europe with a signing ceremony in Warsaw like the one Tuesday in Prague.

"She was going to be traveling in Europe. If there were a possibility of signing it then, that would have been ... positive. If we conclude an agreement, I'm sure that she will look forward to going to Poland at some point," he said.

McCormack declined to completely rule out Rice's making a last stop in the Polish capital following her visits this week to Prague, Sofia and Tbilisi.

Speaking after his meeting with Rice, Sikorski struck a positive note.

"We had some productive ideas and the talks continue," he said.

Asked if an agreement could still be salvaged, Sikorski replied: "There is no need to salvage because talks have continued all along and will continue."

Sikorski was also due to hold talks on missile defense here with US arms control expert John Rood, McCormack added without saying whether they were before or after his meeting with Rice.

The United States wants to base 10 interceptor missiles in Poland and a radar facility in the neighboring Czech Republic by 2011-13 to ward off potential attacks by so-called "rogue" states, notably Iran.

The shield would complete a broader US system already in place in the United States, Greenland and Britain.

A deal under which the Czech Republic would house the radar base was concluded in April, but the negotiations with Poland have proved more complicated.

In Warsaw, Polish foreign ministry spokesman Piotr Paszkowski told that the meeting with Rice had been called at Sikorski's request.

Sikorski was also due to meet with Republican presidential candidate John McCain and speak by telephone with Democratic White House hopeful Barack Obama, Paszkowski said.

Warsaw has refused to budge from its demands for extra security guarantees from Washington if it is to host US missile silos.

"We want to continue this to a happy conclusion. It's about reinforcing our security. We need to consider our interests," Sikorski was quoted as saying by Poland's PAP news agency.

Washington's talks with Warsaw have been grinding on since May 2007.

Warsaw has been lobbying Washington to provide a THAAD or Patriot-type air defense system in exchange for a Polish green light for hosting the silos.

On Friday, Polish Prime Minister Donald Tusk said Warsaw was sticking to its demands for a permanent arms presence guaranteeing Poland's security.

"We can agree anytime, tomorrow, in a week, in a month, as long as we have real guarantees for our security," he said.

Russia is hostile to the idea of having the US missile shield on its doorstep -- and in its communist-era sphere of influence: both Poland and the Czech Republic were Soviet satellite states until 1989.

But the Kremlin has softened its line in recent months and appears to be focusing on getting security guarantees.

NATO, which both Poland and the Czech Republic joined in 1999, endorsed the US plan at a summit in April.

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