The International Monetary Fund said it has become harder to estimate when the Bank of Japan will achieve its inflation target, suggesting the global fund is backtracking from its prediction of around 2017 or 2018.

"It's just too uncertain for us to say x or y time," Kalpana Kochhar, the IMF's mission chief for Japan, said in an interview Friday in Washington. There are many uncertainties, including oil prices, wage growth and companies' potential use of cash holdings, making it hard to make a prediction, she said.

While BOJ Gov. Haruhiko Kuroda reiterated that the bank will attain the goal around the year ending in March 2016, the IMF has forecast that through 2016 inflation will remain halfway toward the central bank's target. A 50 percent decline in the price of oil has halted gains in the BOJ's preferred gauge of prices, making it more difficult for Kuroda to reach his target within the planned two years.