Foreign minister Radek Sikorski made similar comments, saying: "we feel no threat from Iran,", challenging Washington's main argument that the shield is to defend the US and Europe from "rogue states" such as Iran and North Korea.

He added: "it is not only the benefits, but the risks of the system that have to be discussed fully. It cannot be that we alone carry the costs."

The Bush administration wants to deploy ten interceptor missiles on Polish territory and a radar base in the Czech Republic. The military base is expected to be fully operational by 2013.

In response to Mr Tusk's comments, Washington has reaffirmed its aim "to address all of the government of Poland's concerns," while stressing that the missile shield would serve both countries' interests.

"It's in the interests of Poland. It's in the interest of the United States. It's in the interest of other European countries", said US State Department spokesperson Sean McCormack, AP reports.

Mr Tusk's tone stands in stark contrast to that of his predecessor Jaroslav Kaczynski, who was seen as very pro-US and hostile to Russia.

In his first policy speech to the country's parliament last November, the new prime minister said he was set "to convince the US of the need to strengthen Poland's own defences." This could mean an extensive overhaul of its air defences, probably by adding Patriot and THAAD rockets, according to UK daily, The Times.

Some analysts also point out that Warsaw is awaiting new leadership in the White House before taking any firm decision.

"The new Polish administration believes there's no point in pushing ahead with this now, there are only things to lose," said Robin Shepherd, of the UK foreign policy think-tank Chatham House, according to AP.

It remains unclear to what extent Poland's fresh ambivalence will affect Prague, which has been asked by the US to host another section of the same missile system.

The Czech Republic wants to wrap up negotiations on the subject by spring, but domestic polls report that some 70 percent of the Czech population is against the idea.

Several high-level meetings are to deal with the politically sensitive issue.

Later today, Donald Tusk will meet his Czech counterpart, Mirek Topolanek. Next month (8 February), he is also to make his first visit to Moscow, which has long been suspicious of the US missile defence and military base plans for the region.