TOKYO—Japan’s central bank stepped up efforts on Wednesday to keep a key bond yield below a red line, offering to buy more government debt, including through ad-hoc purchases, to hold down interest rates against a pull higher by global yields. The intervention comes as the Bank of Japan (BOJ) seeks to keep monetary policy ultra-loose, even at the cost of fuelling further falls in the yen currency, which could push up import costs and hurt the economy. The central bank increased purchases of Japanese government bonds (JGB) with maturities of three to 10 years, and offered to buy super-long bonds in unscheduled, emergency operations. Combined, the BOJ offered to buy more than 2 trillion yen ($16.40 billion) worth of JGBs on Wednesday, in addition to separate offers for unlimited purchases of 10-year JGBs at a fixed rate of 0.25 percent. “The BOJ is trying to strengthen its message to the …