UNITED NATIONS, March 3 (Reuters) - The United Nations is asking the United States and other nations with big militaries to provide tactical air support for an ill-equipped African Union peacekeeping force in Sudan's Darfur region. With the security situation deteriorating on the ground, the AU force will need more international support in the coming months, even if its mission eventually is to be taken over by a U.N. force, Annan wrote U.S. Ambassador John Bolton in a Feb. 23 letter seen by Reuters on Friday. At the U.N. Security Council's request, Annan has begun planning for a shift from...