Poland's neighbour Russia is opposed to the US shield

In return for hosting part of the shield, the US has said it will help bolster Poland's air defences.

The US wants to install interceptor missiles in Poland and a radar base in the Czech Republic.

Russia opposes the project, saying it would destabilise global security and undermine its own nuclear deterrent.

In October, Russian President Vladimir Putin compared the plans to the Cuban missile crisis of the 1960s, which saw the US and Soviet Union go to the brink of nuclear war.

Russia has threatened to point missiles at Europe if the US positions elements of the new missile shield near its borders.

'Middle of the road'

In response to this threat, Poland says it wants help to upgrade its air defences.

Speaking in Washington, Polish foreign minister Radek Sikorski said he was satisfied that the security concerns Poland had raised would be dealt with.

"There is still a great deal of work for our experts... But yes, I am satisfied that the principles that we have argued for have been accepted," he said.

US Secretary of State Condoleezza Rice said: "We understand that there is a desire for defence modernisation in Poland and particularly for air defence modernisation in Poland. This is something that we support because it will make our ally, Poland, more capable."

America wants to install 10 interceptor missiles to protect against possible attack by what it calls rogue states, such as Iran and North Korea.

But Mr Sikorski said there was still a long way to go.

"We are not at the end of the road as regards negotiations. We are in the middle of the road," he said.

"We have an agreement in principle."

Any final agreement will have to be ratified by the Polish parliament.

The Czech government aims to submit a draft accord on the radar base to parliament in April.