Zimbabwean 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 Thursday 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 forces that 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, Chamisa said.

He said Mugabe, opposition leader Morgan Tsvangirai and the leader of a smaller opposition faction, Arthur Mutambara, would meet again after Mugabe attends the UN General Assembly meeting next week in New York.

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."

UN humanitarian chief John Holmes said aid groups in Zimbabwe have been moving quickly to provide assistance to the large number of needy after the government lifted restrictions on their field operations.

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

He said donor countries had so far provided only 60 per cent of the $394 million US sought by UN officials for humanitarian operations Zimbabwe.

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 viewed 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 run-off against Mugabe. An onslaught of state-sponsored violence against Tsvangirai's supporters led him to drop out of the presidential run-off 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.