The International Monetary Fund said Thursday it would not join a new bailout program for Greece until conditions for debt sustainability, including debt relief and economic reforms, are clearly assured.

"In order to ensure that medium term sustainability, there is a need for difficult decisions on both sides... difficult decisions in Greece regarding reforms, and difficult decisions among Greece's European partners about debt relief," an IMF official said.

Speaking to journalists, the official said the IMF can only support a financial rescue program "that is comprehensive."

"One should not be under the illusion that one side of it can fix the problem," he said.

"What is clear is that it will be some time before the two sides are ready to take these decisions."