* BHP Group BHP.AX plans to almost double exploration spending for base metals within five years, its Chief Technical Officer Laura Tyler said, after shifting its exploration headquarters to Canada. HANOI, June 24 (Reuters) - Copper prices fell on Thursday after comments from U.S. Federal Reserve officials raised fears of a sooner-than-expected interest rate hike in the world's largest economy. A period of high inflation in the United States may last longer than anticipated, two U.S. Federal Reserve officials said on Wednesday, prompting one to pull forward his views on when the central bank should start raising interest rates.