Greece's ruling Syriza party edged toward a formal split on Thursday, hours before rebel leftist lawmakers promised to vote against a new bailout deal to keep the country afloat.

With opposition support, the government is asking parliament to approve a 85 billion euro bailout deal that Greece needs to avoid defaulting on a debt repayment next week.

The parliamentary vote, expected in the early hours of ­Friday, will test the strength of a rebellion by anti-­austerity Syriza lawmakers, which could raise pressure on Prime ­Minister Alexis Tsipras to call snap elections as early as ­September.

The rebels' leader, former energy minister Panagiotis Lafazanis, took a step toward breaking away from Syriza, a coalition of leftist groups which stormed to power in January on promises to reverse austerity policies demanded by the eurozone and IMF creditors.

"The fight against the new bailout starts today, by mobilizing people in every corner of the country," said a statement signed by Lafazanis and 11 other Syriza members posted on the far-left faction's Iskra website.

The statement called for founding a "united movement that will justify people's desire for democracy and social justice" though it did not explicitly call for a new party.

Parliament, however, is expected to approve the bailout agreement by a comfortable margin since opposition parties have promised their backing for the government to ensure Greece does not return to financial chaos.

Tsipras has faced a rebellion among about a quarter of his 149 lawmakers since agreeing last month to the bailout deal under the threat of a banking collapse and eurozone exit.

Earlier, government spokesperson Olga Gerovasili criticized the rebels, saying a government without a majority "cannot go far" and raising the possibility of early elections.

Gerovasili said that after the parliamentary vote, the focus would shift to a meeting of eurozone finance ministers on Friday who must also back the bailout, Greece's third in the past five years.