Greece's debt dynamics are "unsustainable," and the country needs to get serious about reforms if it hopes to fix the situation, according to an analysis released Thursday by the International Monetary Fund.

The fund—a major institutional creditor to Athens—has long called for Greece to adhere to a strict reform program, but the left-wing government has resisted, and ended up missing its June 30 debt repayment of about 1.55 billion euros to the IMF. The institution warned that the country may require a debt writedown without sufficient reform and strong growth.