The US will not recognise the outcome of Friday's presidential election run-off in Zimbabwe, a senior state department official has said.

Jendayi Frazer told the BBC that Robert Mugabe could not claim a legitimate victory amid the current campaign of violence against the opposition.

Opposition leader Morgan Tsvangirai has pulled out of the poll, citing attacks by ruling party militias.

Mr Mugabe has vowed the vote will go ahead, despite international protests.

The opposition Movement for Democratic Change (MDC) says some 86 of its supporters have been killed and 200,000 forced from their homes by militias loyal to the ruling Zanu-PF party. The government blames the MDC for the violence.

Mr Tsvangirai has been taking refuge in the Dutch embassy in Harare since late Sunday, saying he fears for his life. He has called for UN peacekeepers to enter Zimbabwe and protect MDC supporters until a new election can be held.

Ms Frazer, Assistant US Secretary of State for African Affairs, said Washington would not recognise the result of any vote held on Friday because the MDC had been violently forced out of the running.

Her comments came amid growing international condemnation of the political crisis in Zimbabwe.

"People were being beaten and losing their lives just to exercise their right to vote for their leadership so we cannot, under these conditions, recognise the outcome if, in fact, this run-off goes forward," she said.

'Deeply dismayed'

Leaders from the Southern Africa Development Community (Sadc) regional bloc are due to meet in Swaziland on Wednesday to discuss the crisis.

Zimbabwe's opposition is hoping neighbouring countries will put pressure on President Robert Mugabe to step down. In the past they supported him. How are relations now?
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South Africa's President Thabo Mbeki is the key Zimbabwe mediator. He has refused to criticise Robert Mugabe but the ruling ANC and trade unions have urged him to take a stronger line.
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Zambian President Levy Mwanawasa has taken the region's strongest line on Zimbabwe. He says Zimbabwe is a regional "embarrassment".
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Angola's President Jose Eduardo dos Santos is one of Robert Mugabe's closest allies - they fought colonialism together in the 1970s. He has urged Mr Mugabe to stop the violence.
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Botswana has summoned a Zimbabwean envoy to complain about the political violence. It has been supportive of Zimbabwe's opposition.
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Namibia is a close ally of Zimbabwe - it, too, is planning to redistribute white-owned farms to black villagers. It has not criticised the election violence.
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Mozambique has hosted some white farmers forced from Zimbabwe and is seen as relatively sympathetic to Zimbabwe's opposition.
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Tanzania's ruling party has a long history of close ties to Robert Mugabe's Zanu-PF party and its foreign minister has condemned the violence.
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DR Congo's President Joseph Kabila is an ally of Robert Mugabe, who sent troops to help his father, Laurent Kabila, fight rebels.

Malawi is seen as neutral. But some 3m people of Malawian origin are in Zimbabwe, mostly farmworkers who have lost their jobs and were often assaulted during farm invasions.

One of those leaders, Senegal's President Abdoulaye Wade, said the group was pushing for a transitional government in Zimbabwe.

"We will try to install a transition government, a government where responsibilities will be shared," he said on Tuesday after meeting South African President Thabo Mbeki, Sadc's chief mediator for Zimbabwe.

Mr Mbeki has so far been keeping with his policy of "quiet diplomacy" towards Zimbabwe.

South Africa's governing ANC party added its voice to the criticism of Mr Mugabe's government on Tuesday.

In a statement the party said it was "deeply dismayed" that "hard-won democratic rights" were being trampled.

It referred to "compelling evidence of violence, intimidation and outright terror".

The BBC's Peter Biles in Johannesburg says that coming from South Africa - the most powerful country in the region - the ANC statement is a further sign of Mr Mugabe's growing isolation.

Monday saw the UN Security Council make its strongest pronouncements yet against Zimbabwe's government, unanimously agreeing to condemn the violence and saying a free and fair run-off election would be "impossible".

'Protective shield'

But Zimbabwe's veteran leader has shrugged off the criticism.

Speaking at a rally on Tuesday in the town of Banket, Mr Mugabe said: "They can shout as loud as they like from Washington or from London or from any other quarter. Our people, our people, only our people will decide - and nobody else."

Mr Mugabe has defied calls for Friday's vote to be called off

He accused Mr Tsvangirai of pulling out of the election because he became frightened of losing when he saw "a political hurricane coming his way".

Writing in Britain's Guardian newspaper, Mr Tsvangirai says the standard diplomatic approach has failed to resolve Zimbabwe's crisis and calls for a more "activist strategy".

He says UN peacekeepers should "separate the people from their oppressors and cast the protective shield around the democratic process for which Zimbabwe yearns" until new elections can be held under the supervision of African and international monitors.

The MDC won the parliamentary vote in March, and claims to have won the first round of the presidential contest - held on the same day - outright.

According to official results, Mr Tsvangirai was ahead of Mr Mugabe but failed to gain enough votes to avoid a run-off.