Factional leaders are expected to stay in Qatar until they reach a deal
Talks aimed at resolving the Lebanese political crisis appear to have run into further trouble, reports say.
Government and opposition leaders have been in Qatar for several days amid heightened tension following violence in Lebanon earlier this month.
The two sides were considering a plan proposed by Qatar to form a unity cabinet and postpone talks on a controversial draft election law.
But an opposition statement said both issues must be resolved together.
The electoral law could prove decisive in determining results of parliamentary polls due next year.
The conflict has left 65 people dead when opposition militias led by the Syrian- and Iranian-backed Hezbollah movement temporarily seized swathes of west Beirut, ousting armed supporters of the western-backed government.
Correspondents say disagreement over Hezbollah's weapons had already threatened to torpedo the talks when members of the parliamentary majority bloc insisted on debating the issue in the Qatari capital Doha.
Qatari leaders suggested postponing a decision over disputed election legislation and moving directly to a parliamentary vote to name army chief Michel Suleiman as president.
They also proposed forming a unity government of 30 ministers, with 13 ministers from the parliamentary majority, 10 from the Hezbollah-led opposition and seven to be chosen by the elected president.
Rival factions are agreed on electing Mr Suleiman as a president to succeed the pro-Syrian Emile Lahoud, whose term ended in November.
But they have fallen out over power-sharing in a unity government.
The opposition has insisted on holding more than a third of the cabinet portfolios, which would give it power of veto over government decisions.
Lebanon has not had a president since November, when Mr Lahoud stepped down despite parliament failing to elect a successor.
Clashes broke out earlier in May after the government said it wanted to shut down a private phone system operated by Hezbollah and moved the head of security at Beirut international airport for an alleged Hezbollah bias.