A security adviser to George W Bush believes that the Republican administration "has taken a more positive approach than its predecessors to the European Union's attempts to develop its own military capacity".

Writing in a research paper for the Centre for European Reform, Dr Kori Schake - newly appointed to the US National Security Council and a senior research professor at Washington's Institute for National Strategic Studies - argues that EU efforts have "worked to reassure the US that the European Security and Defence Policy (ESDP) will not undercut NATO".

"The terrorist attacks on New York and Washington on September 11th 2001 triggered an outpouring of public sympathy and government solidarity towards the US among its European allies. But the stirrings of a new transatlantic relationship were clear several months earlier, as the rancour that had accompanied the debate over a common European defence policy ebbed away," she writes.

But Schake warns that two serious problems remain over a "divergent" development of the US armed forces and the question of "assured access" for the EU to NATO and American military assets.

"Two serious problems that are threatening the ability and willingness of US and EU forces to work together. Divergence means that US forces are developing in a fundamentally different way than their European counterparts, due to different budgets and policy priorities. This growing dissimilarity is already making joint military operations more difficult, and the gap is increasing steadily," she says.

"Assured access means that a European defence force, operating on its own initiative, could be certain of getting NATO and US military support. However, that expectation may prove unrealistic, which could leave the EU with serious problems in the middle of a crisis".

Cautioning that the "status quo is unsustainable" the Bush adviser believes that divergence between the US and Europe will increase because following September 11 "American defence spending will increase dramatically, the changes occurring in US forces will accelerate, and US interest in and support for crisis-management missions will decline further".

Last year's terrorist attacks may have reinforced European solidarity with the US but Schake predicts that the US-led military campaign in Afghanistan "may yet prove divisive".

"Few NATO allies have the ability to participate, and the US does not want to share intelligence with, or have its operational choices constrained by, states that are not directly involved in the operations," she writes.

To address the problems, she argues, politicians on both sides of the Atlantic should "encourage not only the emergence of a viable European military force, but also the duplication of capabilities already existing in NATO and US forces".

"This approach would allow the EU to conduct military operations without relying on US assistance. It would also increase European influence over US decisions about the use of force. For if EU states are able to participate in the more demanding sorts of combat, their views on how to conduct the military campaign would carry more weight in US decision-making."