The Washington-based lender said Thursday it would not join a new bailout program for Greece until conditions for debt sustainability, including debt relief and economic reforms, were clearly assured.

Briefing reporters on condition of anonymity, a senior official at the Fund said that Greece needed to commit to reforms and creditors must provide debt relief - either in the form of extending loan terms or reducing the debt outright - that would allow the highly-indebted country to pay its bills over time.

"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," the IMF official said.

The official added that the IMF could only support a financial rescue program "that is comprehensive."

He also noted that detailed plans for debt relief would not come for some months during the current first stage of developing the new plan. "They want to see implementation of the policies before they are willing to discuss debt relief in the necessary details," he said, referring to the IMF board members.

Nevertheless, the IMF continues to participate in ongoing debt talks. The fund has said Greece's debts are likely to rise to 200 percent of its economic output in the next two years, a burden the fund calls unsustainable.

Tsipras challenges critics

In Athens, meanwhile, Greek Prime Minister Alexis Tsipras has warned the members of his Syriza party's central committee that the government could fall if not supported by its leftist deputies.

"The first leftist government in Europe after World War II is either supported by leftist deputies, or it is brought down by them because it is not considered leftist," Tsipras said, addressing the party's central committee meeting Thursday.

The party members were meeting to settle differences over whether the government should accept a third bailout from its international creditors.

The deepening crisis within Syriza is the most serious political challenge to Tsipras, who otherwise enjoys unrivaled domination of Greek politics and remains popular despite his sudden U-turn to accept stringent bailout terms.

sri/hg (AFP, AP, dpa, Reuters)