TAIPEI, Taiwan (AP) — U.S. Under Secretary of State Keith Krach has begun the second visit by a high-level American official to Taiwan in two months, prompting a stern warning and threat of possible retaliation from China. Beijing considers Taiwan its own and opposes all official contacts between other countries and the self-governing island. Krach is to meet with Taiwan’s president and hold talks on a trade dialogue. Ahead of Krach’s arrival, the U.S. ambassador to the United Nations had lunch Wednesday with Taiwan’s top official in New York, a meeting the ambassador called historic. Krach’s visit follows one in August by the U.S. health secretary, the highest-level U.S. Cabinet official to visit since the U.S. switched formal relations from Taiwan to China in 1979.