WASHINGTON—Greece may need even more debt relief and financing from its eurozone creditors than the International Monetary Fund estimated less than a week ago, the IMF’s chief economist said Thursday in comments likely to further complicate the country’s efforts to win a new emergency bailout.

In a report published last week, the fund said Greece would need more than €60 billion ($66 billion) in new aid to cover its growing budget shortfalls, as well as substantial debt relief through bond maturity extensions “at a minimum”...