The International Monetary Fund will not take part in a bailout program for Greece if it deems the country's debt is unsustainable, the international lender's chief Christine Lagarde says.

Greece needs to implement reforms agreed by euro zone finance ministers earlier this month to secure a new loan under its 86 billion-euro bailout program, the third since 2010.

The loan is needed to pay debt due in July, but talks continue and the IMF has not yet decided whether to join the bailout. The fund's participation is seen as a condition for Germany to unblock new funds to Greece.

"If Greek debts are not sustainable based on IMF rules and reasonable parameters, we will not take part in the program," Lagarde told German newspaper Die Welt when asked if the IMF would take part in the plan if Greek debt is not restructured.