TOKYO—Struggling to swim against the tide taking interest rates higher globally, the Bank of Japan (BOJ) staunchly defended its 0.25 percent yield cap on Monday by offering to buy an unlimited amount of government bonds for the first four days of this week. The BOJ’s defense of its ultra-loose policy pushed the yen to a six-year low of 124 to the dollar on Monday, adding to the problems Japan’s economy is facing from already surging costs for fuel and raw material imports. Under pressure from a steady rise in yields, the BOJ launched its defense by making two offers in a single day to purchase 10-year Japanese government bond (JGB) in unlimited amounts at 0.25 percent. The central bank then said it would make the same unlimited offer for the next three days, to make sure investors received the message loud and clear, but some economists believed the central bank’s …