The agreement was signed by US Secretary of State Condoleezza Rice and Polish counterpart Radek Sikorski at a ceremony also attended by Polish Prime Minister Donald Tusk and President Lech Kaczynski.

US officials have worked hard to persuade Moscow that the system -- 10 missile interceptors in Poland, placed 185 kilometers (115 miles) from Russia's border, and a tracking radar in Czech Republic -- is not aimed at counteracting Russia's strategic nuclear arsenal.

The timing of the accord has fuelled speculation that Russia's invasion of Georgia, a US ally, spurred US and Polish officials to close the deal after some 18 months of hard bargaining, though diplomats from both countries have denied any link.

Negotiations had, until last Thursday, come to a near standstill on Polish demands that the US reinforce Polish security with Patriot missiles in exchange for hosting the missile shield -- wishes US officials eventually granted.

"The presence of the Patriot battery which will defend our territory ... (while) the U.S. installation is a practical dimension of this watershed agreement," said Polish Prime Minister Donald Tusk, Reuters reported. "Our countries, Poland and the United States, will be more secure."

Russia regards system as affront

Russia has fiercely opposed the missile shield, due to be operational by 2013, saying it would respond by targeting its two former satellite states with a potential nuclear strike.

The decision by Poland, and earlier by the Czech Republic, to host elements of the US shield, "has the Russian Federation as its target," Russian President Dimitry Medvedev told a joint press conference after a meeting in Sochi on the Black Sea last Friday, The International Herald Tribune reported. "This is sad for Europe, sad for all in this densely populated continent."

Russia's deputy chief of general staff, Gen Anatoly Nogovitsyn, said last week the plans for the missile base in Poland "could not go unpunished."

"It is a cause for regret that at a time when we are already in a difficult situation, the American side further exacerbates the situation in relations between the United States and Russia," he said, according to the BBC.

Missile defense simply a safeguard, says Rice

Rice, however, reiterated the missile shield is "purely defensive" and strengthens strategic cooperation with Poland, a former Soviet-bloc nation that was among the first to join NATO after the Cold War ended.

"This is a system that is defensive and is not aimed at anyone," she said. "This is an agreement that will establish a missile defense site ... that will help us to deal with the new threats of the 21st century of long-range missile (attacks) from countries like Iran or North Korea.

"We're talking about a missile defense system that couldn't possibly be aimed at the Russian nuclear deterrent," she told the BBC on Tuesday. "Russia has thousands of nuclear warheads. This is for small missile attacks of the kind that Iran might launch."

Shield accord signals new strength for Poland, Kaczynski

Polish President Lech Kaczynski called the signing an "important day" in Polish history during a televised address to the nation Tuesday night.

Poland had taken a step to "strengthen its position in the world" by sealing the agreement with Washington, he said.

In July, the Czech government agreed to host the system's radar in a military area near the German border.

Approval by the Czech and Polish parliaments is required before the project can go ahead. The system would represent the first US military bases in the two former Warsaw Pact nations.

The US plan had been unpopular in both countries, but a recent poll showed Polish opinion has swung in favor of the shield in the wake of the fighting over the breakaway region of South Ossetia between Russia and Georgia.

Some 58 percent of Poles canvassed said they now supported the Pentagon's plan.

Many Poles view Russia's military incursion into Georgia as proof that Poland needs to bolster its defensive capabilities.