"This is the biggest offset agreement that's ever been signed," Hill said after the contract to buy 48 Lockheed Martin F-16 fighter jets was signed in the central city of Deblin.

"This gives us a real possibility to move ahead in our economic relations," he said.

Under Polish law the cost of procurement contracts must be offset by an investment package.

A statement from the economy ministry and Lockheed Martin said the accord involved over 12 billion dollars of investments.

However, the 12 billion figure appears to be based on the estimated long-term benefits of the investments.

Prime Minister Leszek Miller said at the signing ceremony that the basic value of the investment package would be six billion dollars, but the value could end up being higher.

Poland's choice of the F-16 jets in December, over European rivals, was seen as strengthening Poland's relationship with the United States at a time it approached membership of the European Union.

The relationship deepened even further as Poland put itself behind the US hardline stance on Iraq, sending around 200 soldiers to the Gulf to help the US-led campaign.

While political and military ties have traditionally been strong between Washington and Warsaw, bilateral economic relations have lagged behind.

Hill said the investment package would make it possible for the United States to double the amount of investment in Poland over the next few years.

The United States is currently the second biggest investor in Poland after France, having invested 8.7 billion dollars so far.

Poland needs the fighter jets to come up to the standards of the NATO military alliance which it joined in 1999.

Lockheed Martin snared the contract in December, beating bids from British-Swedish consortium BAE Systems-SAAB with its Jas-39 Gripen and France's Dassault Aviation with its Mirage 2000-5.

The deal drew criticism from EU countries, who thought a member-in-waiting should have given its business to an EU company.

Poland and nine other candidates on Wednesday signed accession treaties to the 15-state bloc and are due to join on May 1, 2004.

SPACE.WIRE