Thousands of opposition supporters have been beaten or worse
UN Secretary General Ban Ki-moon has said the violence in Zimbabwe could undermine the outcome of the presidential run-off election.
"Violence, intimidation and the arrest of opposition leaders are not conducive to credible elections," he told the UN General Assembly in New York.
President Robert Mugabe faces a strong challenge from opposition leader Morgan Tsvangirai in the 27 June vote.
South Africa's president has been attempting to mediate.
President Thabo Mbeki held separate talks with both presidential candidates as pressure mounted on Mr Mugabe to curtail political violence ahead of the poll.
He has made no comment on the progress of his talks on Wednesday.
Official results show Mr Tsvangirai, leader of the Movement for Democratic Change (MDC), won the first round of the presidential election in March but not with enough votes for a clear victory.
Mr Ban called for an end to the violence and for government restrictions on the work of aid agencies to be lifted.
Mr Mugabe has been waging a fierce campaign to extend his 28-year rule
The current political crisis was, he said, compounding an already deep social economic and humanitarian crisis.
"It is of utmost importance that the violence is stopped immediately and that humanitarian assistance is facilitated, not prevented," he added.
A senior UN official, Haile Menkerios, has met President Mugabe to discuss the political stand-off and what the UN says is the increased suffering of an already vulnerable population.
The UN is prepared to pay to fund election monitors to oversee the run-off vote.
On Thursday, US Secretary of State Condoleezza Rice will chair an informal UN Security Council meeting on Zimbabwe in an attempt to maintain international political pressure.
South Africa, which has been leading diplomatic attempts to resolve the situation, is opposed to the Security Council having too much involvement, the BBC's Laura Trevelyan reports from the UN.
Pretoria argues that it is not for the Council to resolve disputed elections.
Earlier, an African poll observer warned that he would not endorse the vote if current levels of violence continued.
Mr Tsvangirai has been detained a number of times while campaigning
Marwick Khumalo, head of the Pan-African Parliamentary observers, told the BBC his team had received horrendous reports of attacks and that the political environment was not conducive to a free poll.
South Africa's foreign affairs ministry said Mr Mbeki's trip on Wednesday to see Mr Mugabe in the Zimbabwean city of Bulawayo was part of his efforts to mediate between the country's veteran president and Mr Tsvangirai.
The BBC's Peter Biles in Johannesburg says this is the third time in as many months that Mr Mbeki has been to Zimbabwe.
But with the vote just days away, there is a growing sense of urgency with political violence beginning to spread from the countryside to the towns, our correspondent says.
However, the opposition Movement for Democratic Change has criticised Mr Mbeki's policy of so-called quiet diplomacy for failing to hold Mr Mugabe to account.
Mr Mugabe has been waging a fierce campaign to extend his 28-year rule since Mr Tsvangirai failed to win enough votes to score an outright victory in March's disputed first round.
Kenyan Prime Minister Raila Odinga has called for an international peacekeeping force to be deployed in Zimbabwe to ensure a free and fair vote.
"It is time for the leaders of Africa to say to President Mugabe that the people of Zimbabwe deserve a free and fair election," he said.
UK Prime Minister Gordon Brown says he has spoken to the leader of South Africa's governing African National Congress, Jacob Zuma, about the possibility of deploying 1,000 election observers from the ANC.
In other developments:
* The MDC's second-in-command, Tendai Biti, who was arrested last week accused of treason, appeared in court on Wednesday in leg irons and was denied access to his lawyers.
* The opposition has said that in contravention of electoral law, it is being denied access to public media controlled by Mr Mugabe's ruling Zanu-PF.
* An election observer was killed in an attack in Karuru, a town north of Harare, early on Tuesday, as his family looked on, a local observer group said
* Local election observers are to be screened by the government to ensure they have "no preconceived ideas" about the vote
* The government lifted a ban on aid agencies which distribute food and Aids treatment