Four members of Zimbabwe's opposition party have been killed in a petrol bomb attack south of the capital Harare.

The Movement for Democratic Change says Robert Mugabe's supporters are stepping up efforts to intimidate them.

New elections take place in the country in just over a week and a group of southern African politicians has joined those voicing doubts Zimbabwe's presidential election will be free and fair.

The Southern African Development Community (SADC) is sending 380 monitors to Zimbabwe for the vote, in which President Robert Mugabe faces the biggest challenge to his 28-year rule from Morgan Tsvangirai, leader of the opposition Movement for Democratic Change (MDC).

Tanzanian Foreign Minister Bernard Membe told a news conference: "There is every sign that these elections will never be free nor fair."

Mr Membe said he and the foreign ministers of Swaziland and Angola would write to their presidents "so that they do something urgently so that we can save Zimbabwe."

Mr Mugabe is accused by opponents, Western countries and human rights groups of orchestrating a campaign of killings and intimidation to keep his hold on the once prosperous country, its economy now in ruins. The MDC claims at least 70 of its supporters have been killed.

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