NATO Secretary General Anders Fogh Rasmussen on Thursday echoed fears voiced by US Defence Secretary Robert Gates that the gulf between US and European funding threatens to weaken the alliance. In comments to The Guardian newspaper, Rasmussen agreed that the disparity in funding would lead to a chasm between the technological capabilities of the US and other member nations, throwing future co-operation into doubt. He highlighted data showing that 10 years ago, US defence investment represented almost half of all defence expenditure in the alliance, whereas today it stood at 75 percent. \"This increasing economic gap may also lead to an increasing technology gap, which will almost hamper the inter-operability between our forces,\" he told the paper. \"The Americans provide still more advanced military assets and equipment; the Europeans are lagging behind. And eventually it will be difficult to co-operate even if you had the political will to co-operate,\" he added. The NATO chief issued a stark warning that unless European partners \"stepped up to the plate\" then the funding disparity would \"weaken our alliance\" in the long-term and create a \"two-tiered alliance\". On Wednesday Gates told a Senate panel that the under-investment by other NATO member states could eventually force the US to leave the alliance. \"The reality is that, as they cut their defense budgets, and... have not been investing in their defense capabilities for a number of years, by default the additional burden falls on the United States,\" Gates said. \"So I think this is a serious problem,\" he said. \"But I think our own financial difficulties and what we\'re now going to face in looking at the American defense budget brings this issue to center stage in a way that it really has not been in the past,\" he added. Gates made similar comments in a speech in Brussels earlier this month.