Japan's biggest banks have warned investors of a tough year ahead.

The nation's three mega-banks have been relying on the healthy status of borrowers and sales of so-called cross-shareholdings for earnings as rock-bottom interest rates crimp lending profitability. Results on Wednesday showed they are losing those benefits at a time when the economy is weakening, trade tensions are escalating and the Bank of Japan's extraordinary monetary easing looks set to stay.

"The business environment is very uncertain and tougher than last year," Sumitomo Mitsui Financial Group Inc. President Jun Ohta said at a news briefing in Tokyo on Wednesday, dismissing the notion that his bank's profit target is conservative.