ATHENS — Greek Prime Minister Alexis Tsipras challenged hard-left rebels in his Syriza movement Thursday to put their opposition to new bailout negotiations to an immediate membership ballot, as he seeks to assert his control over the ruling party.

At a meeting of Syriza’s 200-member central committee held in an old cinema hall with a giant red backdrop, Tsipras defended his decision to accept harsh bailout terms as the best deal anyone could win for Greece.

Greece narrowly averted an exit from the eurozone for now when it struck an Eleventh-hour deal with lenders this month, but that cost Tsipras the support of a quarter of his lawmakers who accuse the party of betraying its anti-austerity roots.

In a defiant speech, the prime minister said his preference was for Syriza to hold an emergency congress in September to deliberate on strategy. If members wanted to challenge his course sooner, the central committee should order a vote by party members in three days.

“We are telling the Greek people, loud and clear and with no remorse, that this is the deal we managed to bring to them and if there is someone who thinks that they could have achieved a better deal, let them come out and say that,” he told the session that included dissenters like parliament speaker Zoe Konstantopoulou.

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.

Failure to quash the far-left revolt could plunge Greece back into turmoil and risk derailing talks now quietly under way in Athens with the European Central Bank, International Monetary Fund and the eurozone’s rescue fund on a new 86 billion euro loan package.

In another potential setback for the talks, the Financial Times reported that International Monetary Fund staff had recommended to the executive board it should not take part for now in a third bailout, because of Greece’s poor compliance record and the unsustainable level of its debt.

Only if Athens improved its track record of implementing reforms and its euro zone partners agreed to a substantial debt restructuring would the IMF consider joining the program at a later stage, the Financial Times report said, citing a confidential summary of a board meeting held on Wednesday in Washington.

An International Monetary Fund official said it would approve new loans for Greece only if Athens reaches a deal with European governments that would ensure it can pay its debts, and there was “no expectation” that talks over the next couple of weeks would get to the point where the Fund could approve a new program.

Greek Finance Minister Euclid Tsakalotos is due to hold his first meeting with senior representatives of the nation’s creditors, including the International Monetary Fund, in Athens Friday.

Germany, Greece’s biggest creditor, has insisted on International Monetary Fund participation in any further aid program.

Damaged relationship

In his speech, Tsipras threw down the gauntlet to Syriza’s Left Platform faction, which has become increasingly vocal in its defiance, by challenging the central committee to call a party member vote for Sunday. He called such a vote a “referendum,” although only party members would be polled, not the general public.

Left Platform leader Panagiotis Lafazanis said the bailout deal betrayed the party’s principles.

“Syriza is at risk of humiliation because of the bailout transformation,” he told the meeting. “In this country there is no democracy… but the dictatorship of the euro.”

As the all-day Syriza meeting proceeded in a rare public session, party officials spoke openly of a widening rift between those who want the country to stay in the euro area despite painful sacrifices, and leftists who would prefer a return to the drachma currency.

“At the moment there are two different strategies competing in the same party — one that wants Greece inside the euro and the other that wants Greece outside euro,” Olga Gerovasili, the government spokeswoman, told reporters as she arrived.

“These two can’t exist together at the government level.”

Deputy Prime Minister Yiannis Dragasakis said the hardliners were behaving like a party-within-the-party, and suggested the time may have come to part ways.

“You reap what you sow and I hope that we are heading for the refoundation of a new party.”

While most speakers criticized the bailout and government actions, many said the left should move on and not waste the historic opportunity of being in power.

Left Platform is demanding Syriza abandon talks with lenders immediately and hold an ordinary party congress to determine the party’s course. A regular congress would favor the far-left, with the same members who attended two years ago returning to vote on Syriza’s future.

Tsipras would benefit from holding an emergency congress instead, which would enable him to bring in new members and capitalize on the wider public support he has secured over the past two years, making it easier to defeat the far-left camp.

A decision between the two options was expected later Thursday evening.

Forced to rely on opposition support in parliament to pass austerity and reforms packages, Tsipras is widely expected to call early elections to consolidate his grip on the party once he has agreed the third bailout package with lenders.

His government wants to wrap up negotiations with lenders on that package in time for a major debt payment due Aug. 20, before moving on to sorting out his party’s troubles.

“Our priority is the [bailout] deal,” Gerovasili said. “After this we can deal with party issues.”