But reports say the displaced villagers face appalling conditions and people are dying every day.

One priest described the sites as ''concentration camps'' and warned people would start ''dying like flies'' unless there was immediate help.

The BBC's Virginia Gidley Kitchin: ''The UN is worried about the next harvest''

The camps are all in the farming province which surrounds the capital, Bujumbura. Many have little or no water, few if any latrines and little shelter.

The army says moving people to camps will help it track down Hutu rebels who in recent months have stepped up their attacks on the city.

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

But correspondents say it is widely believed that the real motive is to prevent them from feeding or sheltering the rebels.

Harvest fears

The UN's World Food Programme (WFP) says it is worried by reports indicating people are being prevented from leaving the camps.

It is particularly alarmed that people may be denied access to their fields during what is now planting season for the next harvest.

A priest at one camp site told the French news agency AFP he had seen thousands of people arrive over the last week.

"They are living in inhuman conditions, with no water or medicine and with nothing to eat. At least five people die every day," he said.

Burundi's civil war, which has claimed around 150,000 lives, pits an army dominated by the Tutsi minority against several Hutu rebel movements.

Most of those who have been "regrouped" are Hutus, who represent some 85% of the total population.

Church massacre

Details of the crisis follow reports of an attack on a church on Sunday in which 30 Catholics were said to have been massacred by uniformed men.

The gunmen opened fire on worshippers in Nyambuye, 55 miles (90km) from Bujumbura, according to the Rome-based missionary news service Misna.

The government of Burundi has denied that any of its troops were involved.

The government blames the recent upsurge in violence on Rwandan militia groups which it says are working alongside Hutu rebel organisations.

The war began after the break down of a power-sharing agreement set up following the assassination of the democratically elected Hutu president Melchior Ndadaye.