The rival parties are supposed share power equally in the new government
Zimbabwean President Robert Mugabe and PM-designate Morgan Tsvangirai are deadlocked over the division of posts in the cabinet, the opposition says.
Mr Tsvangirai's Movement for Democratic Change said discussions were continuing to try to "find common ground".
According to a power-sharing deal, the MDC and a breakaway faction will have 16 ministers in the new cabinet, while Mr Mugabe's Zanu-PF party will have 15.
Earlier, Mr Mugabe said establishing a coalition government was a humiliation.
But the president said Zanu-PF had no alternative after losing the March parliamentary elections.
This would include three posts for one of the MDC factions, whose leader Arthur Mutambara will be deputy prime minister.
MDC spokesman Nelson Chamisa said talks on Thursday had been "inconclusive".
Zanu-PF was "claiming all the powerful ministries" but discussions were continuing, Mr Chamisa said.
"It was a deadlock and has been referred to the negotiating teams for further work to try and find common ground," he told the Reuters news agency.
Before the parties met, an opposition source said Zanu-PF wanted control of powerful portfolios such as finance, defence and information, while the MDC wanted an "equal share".
Under the terms of the power-sharing deal signed on Monday, Mr Mugabe is to retain control of the army.
Mr Tsvangirai is understood to want control of the police by holding the home affairs portfolio.
On Tuesday, Mr Tsvangirai told the BBC that he was working to reassure President Mugabe that he had nothing to fear.
Earlier on Thursday, Mr Mugabe described the power-sharing deal as a "humiliation" that would not have happened if the party had not "blundered" in the March elections.
But he said Zanu-PF nevertheless remained "in the driving seat".
"We are still in a dominant position which will enable us to gather more strength as we move into the future," he said, according to the state-run Herald newspaper.
Meanwhile, the International Red Cross stepping up its distribution of emergency food supplies to Zimbabwe.
Trucks carrying some 383 tonnes of aid for 24,000 people travelled through the night after loading supplies in Harare, Bulawayo and Mutare.
Red Cross spokesman Matthew Cochrane told the BBC that some two million people needed food aid, and this could rise to five million - half the population - by the end of the year.
"The situation is critical," he said.
The new government has said its first priority is to try to revive the ruined economy.
Annual inflation is running at an official rate of 11,000,000% and just one adult in five has a regular job.