Zimbabwe's President Robert Mugabe said on Friday liberation war veterans would take up arms if he loses a June 27 presidential run-off vote.

Mugabe told youth members of his ruling ZANU-PF party in Harare that the veterans had told him they would launch a new bush war if the election was won by opposition leader Morgan Tsvangirai, whom he accuses of being a puppet of the West.

"They said if this country goes back into white hands just because we have used a pen (to vote), 'we will return to the bush to fight,'" Mugabe said, in the latest racketing up of pressure to extend his 28-year-presidency.

Tsvangirai, human rights groups and Western powers accuse Mugabe of unleashing a brutal campaign to win the run-off after he lost presidential and parliamentary elections on March 29.

Tsvangirai fell short of the majority needed to win the presidency outright in that vote. He says 66 of his followers have been murdered since.

But former guerrilla commander Mugabe, president since independence from Britain in 1980, blames the MDC for the violence which has caused widespread international concern.

"We cannot allow the British to dominate us here again through their puppets. You saw what they were saying (after the March elections), celebrating an MDC victory," the 84-year-old ruler said on Friday.

"These were the whites we took farms from."

The war veterans, usually acting alongside the ZANU-PF youth militia, have regularly been used to intimidate Mugabe's opponents and were involved in implementing the government's seizure of thousands of white-owned farms beginning in 2000.

Some of the seized land was given to the veterans.

Earlier, the MDC said Zimbabwean police impounded two campaign buses used by Tsvangirai in the latest action against the opposition leader in the election campaign.

Tsvangirai, who has been detained four times in the past week and has had his own vehicle confiscated, would continue the campaign, MDC spokesman George Sibotshiwe said.

MDC Secretary General Tendai Biti, who was arrested on Thursday, remained in police custody facing a treason charge that could carry a death sentence. His lawyers filed an application asking the High Court to intervene.

INTERNATIONAL CONCERN

The Southern African Development Community, a grouping of 14 nations including Zimbabwe, has sent a team of election monitors to Harare. Observers from Western nations critical of Mugabe's government are not being allowed into the country.

Zimbabwe's agricultural sector, once one of the most prosperous in Africa, has collapsed, and shortages of bread, milk and meat are common. Inflation is running at 165,000 percent and unemployment is 80 percent.

U.S. humanitarian affairs chief John Holmes said the situation was deteriorating rapidly. He called it "very worrying and very serious with up to 4 million people in need of humanitarian assistance".

The government last week banned foreign aid groups and a regional rights group said on Friday domestic non-governmental organisations had also been ordered to stop working.

The South African Litigation Centre (SALC) said police had ordered several NGOs to close, including human rights groups and the Zimbabwe Women Lawyers Association.

Washington has called for urgent U.N. Security Council talks on Zimbabwe. But diplomats said South Africa, supported by China and Russia, was opposing this.

South African Deputy Foreign Minister Aziz Pahad said on Friday his government would do everything possible to prevent a civil war.

"A civil war will not be in the interests of the region and so we will do everything possible, firstly to deal with all the reports of the escalating violence and secondly to make sure that we never reach the possibility of a civil war because that would be a disaster not only for Zimbabwe but for all of us," he told reporters.

A group of prominent African leaders joined calls for an end to the violence.

"It is crucial for the interests of both Zimbabwe and Africa that the upcoming elections are free and fair," former U.N. Secretary-General Kofi Annan and 39 former African heads of state and civic leaders said in an open letter on Friday.