NEW YORK (MarketWatch) -- International Monetary Fund officials are threatening to suspend aid payments to Greece by the end of June unless euro-zone leaders move to plug a gap of 3 billion to 4 billion euros ($4 billion to $5.3 billion) in the country's rescue program, the Financial Times reported Thursday, citing officials involved in managing the bailout. The gap stems from euro-zone central banks refusing to roll over Greek bonds, the report noted. Officials emphasized that Athens isn't to blame, the report said, noting that one official said the shortfall may force euro-zone finance ministers to discuss "alternate sources' of funding, including the potential for a new bailout program by the end of the year.