Johannesburg - Former UN secretary general Kofi Annan, former US president Jimmy Carter and Mozambican activist Graca Machel had to call off a trip to Zimbabwe after President Robert Mugabe refused them entry, they announced in Johannesburg Saturday.

Addressing a press conference, the three, who were due to travel to Zimbabwe Saturday on behalf of The Elders grouping of leading statesmen and women, said they were disappointed that they had been denied an opportunity to shine a light on the humanitarian crisis in the country.

South African President Kgalema Motlanthe and ex-president Thabo Mbeki, who is mediating in Zimbabwe's power-sharing talks, had intervened on their behalf with Mugabe, but to no avail, they said.

"It seems obvious to me that the leaders of the (Mugabe) government are very immune to reaching out for help for their people," Carter said.

Machel, a well-known social rights campaigner and wife of The Elders convener, anti-apartheid icon Nelson Mandela, also said she was "extremely disappointed."

This is the first time The Elders - which was founded in 2007 to tackle conflicts around the world and also includes South African Archbishop Emeritus Desmond Tutu, an outspoken critic of Mugabe - has been refused entry to a country.

A state newspaper in Zimbabwe on Thursday quoted a government source denigrating the visit as a "rescue package" for the opposition Movement for Democratic Change and "a partisan mission by a group of people with partisan interests." The source had also claimed the government was "not in a position to handle the visit at this time of the year."

The aborted visit came as Zimbabwean police reported a powerful bomb had exploded in Harare's main police station on Thursday night, the second in a week to strike at senior police offices.

No-one was injured in the blast that shattered walls and windows in the criminal investigation department (CID, police assistant commissioner Wayne Bvudzijena said.

On Monday night, a first bomb exploded in the headquarters of the same department in the main police base in Harare, also without causing injuries.

Buvdzijena said no arrests had been made in connection with any of the explosions. State media has speculated that they were aimed at destroying files in high-level fraud and corruption cases.

The Elders had been due to meet with aid organizations and health workers to discuss a worsening food crisis and a spiralling cholera outbreak that has claimed nearly 300 lives and infected thousands of others, according to several diplomatic and medical sources.

State media said seven members of a local Christian sect were among the victims of the water-borne disease in the Harare township of Budiriro this week. They had refused treatment on the grounds their religion bans them from all medication other than "holy water."

Two adults and three children had died by the time police forced the parents of another two children to take them for medical attention, but by then it was too late, the daily Herald newspaper said.

While stressing their focus was on humanitarian needs, including the food shortages suffered by over 3 million people, Annan, Machel and Carter had also sought meetings with Mugabe and the opposition.

They had been scheduled to meet with MDC leader Morgan Tsvangirai in South Africa on Friday. It was not clear whether the meeting took place.

The MDC is coming under pressure from Zimbabwe's neighbours to help stem the crisis by accepting Mugabe's terms for joining a unity government, in which Mugabe remains president and Tsvangirai becomes prime minister.

The MDC, which won the last parliamentary elections, accuses Mugabe's Zanu-PF of keeping all the important cabinet portfolios, bar finance, for itself.

The parties are reportedly scheduled to meet again next week under Mbeki's mediation to discuss a draft constitutional amendment that will set out Tsvangirai's powers as prime minister relative to Mugabe's. (dpa)