The Washington-based Congressional Budget Office has urged Nato in a report to make its own estimate of the costs, saying: "These costs could be substantial and for that reason deserve analysis."
Nato officials say they have not made detailed calculations of the costs associated with expansion. They say they cannot do so until the alliance has identified which countries in central and eastern Europe will join.
However, it is an open secret that the Czech Republic, Hungary, Poland and possibly Slovakia - the so-called Visegrad states - are the most likely candidates for membership. Based on that assumption, the congressional study estimates that the total cost of enhancing Visegrad defences and boosting Nato's ability to reinforce the four countries would be $60.6bn over 15 years.
If the Visegrad states received no Western funds to assist their integration, it would cost them $42bn to join the alliance, while the US would pay $4.8bn and other Nato members $13.8bn. However, since it seems doubtful that the Visegrad countries could afford such sums, the study estimates that the US would take on $21bn of their expenses, leaving the Visegrad states with $21bn to find over 15 years.
The report says the total cost of enlargement could double to $124.7bn over 15 years if, in addition to the above measures, Nato projects air power eastwards to defend the Visegrad states, strengthens Nato's ground forces in Germany, moves equipment stocks to Visegrad territory and stations a limited number of forces forward. Under the compromise being discussed, it appears that Nato would refrain from taking some of these steps, keeping the cost below $124.7bn.
It is also uncertain that all four Visegrad states will be simultaneously incorporated into Nato. The US and the European Union have expressed doubts about democracy inSlovakia since the populist Prime Minister, Vladimir Meciar, returned to power. in October 1994.