The yen sank to a 34-year low against the dollar Thursday after a top Bank of Japan official indicated it would press ahead with a loose monetary policy, even after this month hiking interest rates for the first time since the global financial crisis.Earlier in the day, Bank of Japan board member Naoki Tamura said officials would not embark on a speedy programme of monetary tightening as they try to nurture an economic recovery while keeping a lid on inflation.