President Robert Mugabe and his political rivals have been unable to agree on how to share key Cabinet posts, an opposition spokesman said Thursday in a sign that deep and bitter divisions were threatening a watershed unity government agreement.

Nelson Chamisa of the Movement for Democratic Change said a meeting of party leaders broke up with no resolution and deputies had been asked to keep negotiating.

Mugabe's party "is claiming all the powerful ministries," Chamisa said. "That is why there couldn't be agreement and it's being referred back to the negotiators."

He said the ministries in contention included home affairs, which directs police who have been accused of political violence. Mugabe remains commander in chief, so the opposition was likely to insist on control of at least some security forces.

The other disputed ministries were foreign affairs, finance and local government, he said.

Chamisa said Mugabe, opposition leader Morgan Tsvangirai and a leader of a smaller opposition faction, Arthur Mutambara, would meet again after Mugabe attends the U.N. General Assembly meeting next week.

Thursday's talks were the first since the signing of a power-sharing deal on Monday that has Mugabe ceding some power for the first time in 28 years.

Under the pact, Mugabe remains president and head of government, chairing the Cabinet. Tsvangirai will be prime minister and head of a new Council of Ministers responsible for forming government policy. He is deputy chairman of the Cabinet.

The agreement provides for 31 ministers — down from 50 — 15 nominated by Mugabe's party, 13 by Tsvangirai and three by Mutambara.

Many Mugabe loyalists will lose their jobs. Mugabe told his party leaders at a meeting Wednesday that sharing power with rivals is a "humiliation" that has to be accepted because they lost March elections.

The South African government, whose President Thabo Mbeki brokered the deal during weeks of negotiations, said Thursday it would keep working "to ensure that the agreement is implemented." South Africa said Zimbabwe's politicians need to move on to solving their country's economic crisis.

"The agreement represents the beginning of a process of restoring peace and stability for the people of Zimbabwe," a South African Cabinet statement said. "The next steps include addressing the challenge of beginning with the enormous task of rebuilding the economy of Zimbabwe."

Meanwhile, U.N. humanitarian chief John Holmes said the aid community in Zimbabwe is moving quickly to provide assistance to the large number of needy people, after restrictions were lifted on the field operations of NGOs.

"This is a critical moment," Holmes said in a statement issued at U.N. headquarters in New York. "The people of Zimbabwe urgently need food, seeds, fertilizers and essential drugs, among so many other priorities."

He said donors have provided only 60 percent of the US$394 million the U.N. is seeking for Zimbabwe, and he urged donor nations to step up their contributions.

"The government of Zimbabwe must ensure safe, unfettered access by the humanitarian community as it undertakes its critical work," Holmes said.

Western governments poised to help with aid and investment have so far stayed on the sidelines, waiting to see whether Tsvangirai will emerge as the main decision-maker. The United States and Britain have been among Mugabe's sharpest critics, accusing him of human rights abuses and ruining the economy.

The West's attitude has been seen as one reason Tsvangirai should name the finance minister. Tsvangirai is seen as being capable of reassuring the West that aid money would not be wasted or stolen.

Tsvangirai won the most votes in March presidential polling, but not enough to avoid a runoff against Mugabe. An onslaught of state-sponsored violence against Tsvangirai's supporters led him to drop out of the presidential runoff and Mugabe was declared the overwhelming winner of the second vote, which was widely denounced as a sham.

A resurgence of political violence appeared unlikely, despite the tensions caused by delays in implementing the power-sharing deal. Zimbabwe has been largely calm since June, and the political leaders all have pledged to make the agreement work.