PRETORIA (Reuters) - U.N. Secretary-General Ban Ki-moon on Wednesday called on Zimbabwe's President Robert Mugabe to release detained activists and said such a move would help unlock international humanitarian support.

Many of the activists still held in Zimbabwe are members of Prime Minister Morgan Tsvangirai's Movement for Democratic Change, the long-time opposition party which entered a unity government with Mugabe earlier this month.

One of the most prominent is senior MDC official Roy Bennett, who remains in detention pending an appeal by state lawyers against the high court's granting of bail on Tuesday.

It would be a welcome gesture for the leader of Zimbabwe to embrace all different opinions and leaders in the country by releasing all these detained people, Ban said.

I hope that he would listen to the expectations of the international community by releasing them all as soon as possible, he said after talks in South Africa with President Kgalema Motlanthe.

Bennett, a white farmer who lost his farm under Mugabe's land seizures, had been earmarked to become deputy agriculture minister. He faces charges -- which he denies -- of plotting terrorism, insurgency and banditry.

Zimbabwe's new government urgently needs to tackle an economic meltdown that has led to the world's highest inflation, food shortages and a cholera epidemic that has killed 3,877 people and infected over 83,000 others.

Tsvangirai said last week it would cost as much as $5 billion to repair the economy.

The economic situation is very dire and the humanitarian situation is also very worrisome, Ban said.

RECONCILIATION

He added that Mugabe should promote national reconciliation and the international community, led by the United Nations, stood ready to help support Zimbabwe's recovery with humanitarian aid.

But all these efforts ... would get stronger and more support from the international community if we can see progress in the political and national reconciliation, Ban said.

The 15-nation Southern African Development Community said a regional aid package for Zimbabwe could be affected by the impact of the global financial crisis.

SADC finance ministers were meeting in South Africa on Wednesday to discuss an aid package for Zimbabwe.

When (finance) ministers consider any support to Zimbabwe they have to take into consideration what is going on (globally), so for sure it is a challenge, SADC Executive Secretary Tomaz Salomao told Reuters.

The global economic meltdown has had a negative impact on the SADC -- with many of the bloc's members net importers of fuel or food -- slowing GDP growth and increasing current account deficits, even in South Africa, the bloc's strongest economy.

In Harare, Tsvangirai said the country's new coalition government was gathering momentum, but some issues were unresolved.

Asked if he could still trust his old foe Mugabe, Tsvangirai told a news conference: I was sworn to respect President Mugabe as head of state ... there are some teething problems that we're facing, but I'm sure he understands we can work together and (he) is committed.

But Tsvangirai still appears guarded, suggesting that Mugabe was trying to create power structures to weaken his government.

This government will not allow a parallel force within its structures or any unconstitutional or unilateral actions which serve to impede progress, he said in a statement.

Investors and Western donors are looking for concrete signs of political stability and bold economic reforms before they pour in money to help rebuild once prosperous Zimbabwe. Substantial financial support will be unlikely until all outstanding issues are tackled.

As long as these matters remain unresolved, it will be impossible for the transitional government to move forward with the reforms that this country so desperately needs, said Tsvangirai.

(Additional reporting by Nelson Banya in Harare; Wendell Roelf in Cape Town; Writing by Marius Bosch; Editing by Giles Elgood)

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