The International Monetary Fund on Tuesday unexpectedly upgraded its projections for Japan's economic growth this year and 2016, pointing to a weak yen and lower oil prices, but also warning that Tokyo must tame its massive national debt.

In its latest World Economic Outlook, the Washington-based IMF said it expects Japan's gross domestic product to expand by 1.0 percent in 2015 and by 1.2 percent in 2016, lifting projections earlier this year for 0.6 percent and 0.8 percent growth respectively.