AS THE US expands its involvement in Afghanistan, military experts are warning that it is taking on security and political commitments that will last at least a decade and a cost that is likely to eclipse that of the Iraq war.

This assessment follows comments on Saturday from the new head of the British Army, General David Richards, who believes stabilising Afghanistan may take as long as 40 years.

Since the invasion of Afghanistan in 2001 the US has spent $US223 billion ($267 million) on war-related funding for that country, according to the Congressional Research Service. Aid spending, excluding the cost of combat operations, has grew from $US982 million in 2003 to $US9.3 billion last year.

The costs are almost certain to keep growing. The US is in the process of overhauling its approach to Afghanistan, focusing on long-term security, economic sustainability and development. That approach is also likely to require deployment of more US military personnel.

Later this month the top US commander in Afghanistan, General Stanley McChrystal, is expected to present his analysis of the situation in the country.

Military experts, including some advisers to General McChrystal, insist that additional resources are necessary. ''We will need a large combat presence for many years to come, and we will probably need a large financial commitment longer than that,'' said Stephen Biddle, a senior fellow for defence policy at the Council on Foreign Relations and a member of the ''strategic assessment'' team advising General McChrystal.

The expansion of the Afghan security force that the general will recommend ''will inevitably cost much more than any imaginable Afghan government is going to be able to afford on its own'', Mr Biddle said.

''Afghan forces will need $US4 billion a year for another decade, with a [similar] sum for development,'' said Bing West, a former assistant secretary of defence who has chronicled the wars in Iraq and Afghanistan.