A UN spokesperson said the suspension was intended as "a mark of respect" to the two officials and would last for three days.

BBC's Chris Simpson in Bujumbura: UN says "no retreat" form Burundi

"We will most likely resume activities next week if we feel we can put in place the necessary measures to guarantee the security of our staff," said Michele Quintaglie of the World Food Programme.

The move is in response to the murder of nine people - including two senior United Nations officials - in an attack on an humanitarian convoy in the south-east of the country.

Burundi Government spokesman Appollinaire Gahungu: "The rebels hid among civilians"

The dead included the UN Children's Agency representative in Burundi, Luis Manuel Zuniga, from Chile, and 34-year-old World Food Programme logistics officer Saskia Von Meijenfeldt, from the Netherlands.

Ms Quintaglie said six members of the UN team had been put against a wall shortly after arriving at the camp and robbed by the rebels, who then walked away.

"One rebel turned around and said 'Why should we let these people live?'," she said.

"He took his gun, put it to the head of the UN worker and executed him. Then he took his gun again, put it to the head of the WFP worker, and executed her."

Seven people are reported to have been injured and three vehicles destroyed.

A government spokesman, Appollinaire Gahungu, said the incident happened as the convoy visited a civilian camp at Muzye, 140km south-east of the capital Bujumbura.

Mr Gahungu said soldiers travelling with the convoy were unable to respond because the attackers were hidden among the civilians.

"There was a risk of hitting many people, "he said.

The government is blaming rebels operating from neighbouring Tanzania.

Ultimate sacrifice

The killings brought an angry response from the UN Secretary-General Kofi Annan.

UN spokesman Manoel de Almeida e Silva: "We mourn the loss of irreplaceable colleagues"

In a statement read by his deputy spokesman, Mr Annan said that the UN officials had made the ultimate sacrifice in the cause of peace.

Paying tribute at the same time to a UN worker killed in Kosovo this week, Mr Annan said the officials were dedicated to creating an environment "where innocent victims will no longer be the first victims of conflict".

"Let us insist that the killers be brought to justice," he said.

A BBC Correspondent in Burundi, Chris Simpson, says the UN has a high profile in the country and is running many aid projects throughout the region.

Upsurge in attacks

The six-year civil war in Burundi has pitted the mainly Tutsi-dominated army against Hutu rebel groups operating from bases in the Democratic Republic of Congo and Tanzania.

Pierre Buyoya: Under pressure from rebels

More than 200,000 people - most of them civilians - have died in the conflict.

Several Hutu rebel organisations are trying to oust the government of President Pierre Buyoya - a continuation of a cycle of violence that erupted after the assassination of the country's first democratically-elected president, Melchior Ndadaye.

In recent weeks, there has been an upsurge in rebel attacks on Bujumbura, prompting the government to set up controversial "regrouping" camps, ostensibly to protect civilians from attack.

UN officials say that in the past few weeks, the army has forcibly moved 260,000 civilians into makeshift camps.

The displaced villagers face appalling conditions and people are dying every day, local reports say.

Many of the camps have little or no water, few, if any, toilets and little shelter.

Protection from crossfire

The army says moving people to camps will help it track down the rebels.

It says it is simply trying to protect the civilians from the violence or from getting caught in crossfire.