Donors expected to pledge $16 billion in Afghan aid KABUL (Reuters) - Donors are expected to pledge $16 billion in aid for Afghanistan over the next four years, a U.S. official said on Saturday, as Washington kept a promise to declare the country a major non-NATO ally. The upgrade in Afghanistan's security status, a largely symbolic move for now, and a donors' conference to be held in Tokyo on Sunday both aim to reinforce the U.S. message to Afghans that they will not be abandoned as the war winds down. The new status may help Afghanistan acquire U.S. defense supplies and have greater access to U.S. training as the Afghan army takes more responsibility for the country's security ahead of the 2014 withdrawal of most NATO combat troops. Donor fatigue and war weariness are taking their toll on how long the global community is willing to support Afghanistan and there are fears that without financial backing, the country could slip back into chaos when foreign troops withdraw. Please know that the United States will be your friend and your partner. We are not even imagining abandoning Afghanistan. Quite the opposite," U.S. Secretary of State Hillary Clinton told a news conference with Afghan President Hamid Karzai during an unannounced visit to Kabul before flying to Tokyo. The decision, formally taken by U.S. President Barack Obama, keeps a promise that he made on a visit to Afghanistan this year to upgrade Kabul to a special security status given to only a limited number of U.S. partners - including Israel and Japan - which are not members of the NATO alliance. However, it is not expected to have any immediate practical effect, a U.S. official said on condition of anonymity, saying the large presence of NATO forces in the country already confers some similar benefits. Participants at Sunday's donors' conference in Tokyo are expected to pledge $4 billion annually in development aid for Afghanistan over the 2012-2015 period, a U.S. official told reporters as Clinton flew to Japan. The figure is well below the sum of at least $6 billion a year that the Afghan central bank has estimated is needed to foster economic growth over the next decade. The development aid would come on top of the $4.1 billion committed annually by NATO and its partners for Afghanistan's security forces, pledged at a Chicago summit in May. U.S. officials with Clinton declined to say how much economic assistance would be pledged by Washington, which has significantly reduced aid since the peak year of 2010 when it provided two-thirds of the more than $6 billion given. The United States will be making a substantial commitment in line with what we have been providing in the past. We want to continue at or near that level," Clinton said in Kabul without elaborating. A U.S. official said Washington had given $2.3 billion in the current U.S. fiscal year that ends on September 30, while a decade ago the amount was closer to $1 billion. U.S. officials acknowledged that the trend lines for development aid were now heading down. Major donors and aid organizations have warned that weak political will and corruption could prevent funds reaching the right people at a critical time, when fragile gains in health and education could be lost. For Afghanistan to one day stand on its own two feet, donors hope Sunday's Tokyo conference will win guarantees from Kabul to tackle graft and ensure accountability. Transparency International ranks Afghanistan as one of the world's most corrupt nations and despite Karzai's recent campaign to tackle graft within government, a high-level corruption case is yet to be prosecuted. There are also growing fears among ordinary Afghans that a vacuum of cash could lead to greater insecurity. Comparisons are frequently being drawn to the Soviets' 1989 humiliating defeat after a decade of fighting mujahideen insurgents. When Moscow stopped giving aid a year after the Soviet Union collapsed, in 1992, civil war engulfed the country. U.S. officials may be reluctant to cite a specific pledge because the sum given is ultimately controlled by Congress, which holds the U.S. government's purse strings. Enthusiasm for foreign aid has waned in Congress because of massive U.S. budget deficits. Writing by Amie Ferris-Rotman and Arshad Mohammed; Editing by Jeremy Laurence and Pravin Char