Botswana says its President Seretse Khama Ian Khama will boycott a summit of regional leaders because Zimbabwean leader Robert Mugabe has been invited.
The country has said that Mr Mugabe should not attend such gatherings until a power-sharing deal has been reached.
It is also urging its neighbours not to give legitimacy to the widely-condemned Zimbabwean presidential elections.
South African President Thabo Mbeki, who is mediating in the Zimbabwean talks, is hosting the summit.
Correspondents say Botswana's move to boycott the 14-member Southern African Development Community (Sadc) summit is "unprecedented" and add that it shows growing opposition to EU Supports Pretoria's Mediating Role in Zimbabwe ...
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Meanwhile, UN Secretary General Ban Ki-moon has called on the Zimbabwean government to lift restrictions on aid deliveries, to stop "a catastrophic humanitarian crisis".
"I call on the government of Zimbabwe to fully respect humanitarian principles and the impartiality and neutrality of voluntary and non-governmental organisations, allowing them to operate freely and with unrestricted access to those in need," Mr Ban said in a statement.
Condemned
He said curbs on aid agencies imposed in June meant that less than 20% of 1.5m people in need had received help.
Talks in Zimbabwe have involved President Robert Mugabe's Zanu-PF party, Morgan Tsvangirai's Movement for Democratic Change (MDC) and a breakaway MDC faction led by Arthur Mutambara.
The BBC's Letlhogile Lucas in Botswana's capital, Gaborone, says the government took the decision because it did not recognise Mr Mugabe as the legitimate president of Zimbabwe following his victory in an election that was widely condemned as a sham.
Mr Tsvangirai won the first presidential round in March, before pulling out of a June run-off citing a campaign of violence against his supporters.
Basic foodstuffs, including maize meal and bread, are often in short supply in Zimbabwe, which was once one of Africa's leading agricultural producers.
About 80% of the country's 12.3m people are unemployed and many depend on food aid.
Impact on neighbours
Prior to the food aid ban, many Zimbabweans were already suffering from food shortages and rampant inflation, a situation made worse by the election violence, the UN said.
The government had accused aid agencies of campaigning for the opposition.
Many aid agencies have pulled staff out of rural areas since field operations were frozen by the government.
Earlier this week, Mr Mbeki spoke about Zimbabwe's economic crisis.
Hundreds of thousands of Zimbabweans have fled the worsening political and economic situation, many crossing over the borders into neighbouring South Africa, Zambia and Botswana.
Mr Mbeki said he was determined to reach a deal and promised to stay in Zimbabwe for six months to get one if he had to.
Mr Mugabe has blamed the crisis on a Western conspiracy to remove him from power.
(BBC)
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