The new head of the nation's main banking lobby is warning the Bank of Japan against deepening negative interest rates, signaling that such a move could spur risky investment and put further pressure on lenders' profits.

"It will be a quite difficult option to take," Makoto Takashima, chairman of the Japanese Bankers Association, said in an interview. "Simply speaking, that would cause policy side effects to further grow."

Speculation for more BOJ easing has resurfaced as the economy weakens and central banks around the world pivot away from policy tightening.