August - the killing is still going on. It is impossible to count the dead but some put the figure at more than one million. Most of them have been murdered with machetes, spears, knives, even hoes. In Rwanda there are almost no Tutsi civilians left alive. The rebel movement, the Rwanda Patriotic Front (RPF), which is mainly Tutsi, drove the mainly Hutu government forces out of the capital in May and then took over the rest of the country.

This sent the defeated government army and its Hutu militias into a new frenzy of killing and the RPF was stretched too thinly to stop them. Its military victory is turning to ashes. The Tutsis have all been massacred and the Hutus have fled from the RPF into neighbouring countries. Hutu militias are launching a guerrilla war against the RPF and the renowned discipline of its soldiers is cracking as they make reprisals.

By the end of August there are two million Rwandans in exile, a quarter of the population. Most of them are Hutus afraid to return to their homes in areas ruled by the RPF. They try to survive in vast camps over the border in Tanzania, but the camps are inaccessible and UN blunders and bureaucracy are turning a disaster into a catastrophe. The death rate in the camps is the worst ever recorded.

But Rwanda's genocide is overshadowed by events in its twin state, Burundi. In late May thousands of Rwandan Tutsis fled across the border to seek the protection of their fellow Tutsis in Burundi. There the local Hutu people felt threatened by the influx into their already crowded land and Tutsis were killed. In reprisal the Burundi army, which is exclusively Tutsi, started killing Hutus in the capital Bujumbura. By June this had sparked another exodus of refugees into Zaire and Tanzania. Finally, in Zaire Rwandan exiles have become embroiled in war with the local people and hundreds of thousands are being killed.

The UN, already overstretched, is unable to protect or feed any more refugees. In the vast unmanageable camps in Tanzania and Uganda the death toll is about 10,000 a day, and many more refugees are cut off in Zaire. It is the worst human catastrophe of the late 20th century. How much of it we will be shown is another matter. In early May, television news editors were already telling their crews that they wanted no more pictures of bodies.

This is a nightmare vision of the future but it is no fantasy, given what has occurred in Rwanda, Burundi and Zaire in the past year. So far the only blessing is that the three countries have not exploded at the same time. But now Burundi is coming to the boil again. Foreigners fleeing Bujumbura this week warned that the country could explode again at any moment. United Nations staff have warned their New York headquarters that the whole of Central Africa could be engulfed in ethnic pogroms on a scale worse than anything seen so far.

There is no UN contingency planning for this possibility. The United Nations High Commission for Refugees has launched a dollars 60m appeal for the 250,000 Rwandan refugees in Tanzania. It is unlikely to reach that target. There is no chance that a peace-keeping force will be available or able to stop a disaster.

What happened in Burundi last year and Rwanda last month should be no surprise. In Burundi there were massacres in 1965, 1972, 1988 and 1991. The biggest occurred last year. Rwanda saw mass killings in in 1959, 1961 and 1963. With a history like that it was nave to imagine that moves towards democracy and free-market economics (encouraged throughout Africa by Western governments) could be managed peacefully.

It is difficult for outsiders to understand why people can turn on their neighbours with such appalling ferocity. People in both countries talk of fear but few talk of hatred. There are many Hutus and Tutsis who live happily and peacefully together. In fact, the genocide in Burundi was promoted by Tutsi leaders afraid of losing their power and privilege and Hutu leaders in Rwanda afraid of a return of Tutsi domination. The late President Juvenal Habyarimana of Rwanda, whose assassination sparked the massacres, helped to set up the Hutu militias ready for the final cleansings, or nettoyage, as Hutu extremists call it.

The killers here speak of solving the problem once and for all. It is an African Final Solution. And it was planned. In the lush green hills there was no spontaneous uprising of ethnic hatred but a pogrom planned and paid for. The organised gangs of Hutu killers combed the crowded hillsides for every Tutsi - and any Hutu suspected of supporting political parties which favoured a peace agreement. They slaughtered men, women and children in their thousands.

The planned extermination of so many by so many, by hand, over such a long period suggests some deep unconscionable mass psychosis. Not history, nor the ethnic differences, nor the crowding, nor the political encouragement seem sufficient to explain the comprehensiveness or the barbarity of the killings, but they indicate that having gone so far they are more likely to spread than to stop now.

Nor can the mass exterminations in Rwanda and Burundi simply be dismissed as another ghastly example of African tribalism. These two former kingdoms are unique, and the Hutu and Tutsi people cannot be defined as tribes. They speak the same language, share the same culture, live on the same hills and in recent years began to intermarry. These densely populated kingdoms in the heart of Africa were first colonised by the Germans and then mandated to the Belgians after the First World War. The colonial powers kept and strengthened the hierarchical structures of society. In both cases the Tutsi were a kind of feudal cattle-owning aristocracy who lorded it over the Hutu peasants. Belgian rule brought the added advantage of education for the Tutsi, and at independence the they enjoyed a preponderance of professional jobs, even though in both countries they form only about a tenth of the population.

The Tutsis still look and act the part. The stereotype - tall, brown skinned, fine featured - is clearly distinguishable from the shorter, darker, Hutu. The Tutsis are often arrogant towards strangers as well as their Hutu countrymen. But what was once an ethnic and feudal division has now become a class division. Tutsis have adapted better to Western civilisation. They tend to have smaller families and ensure that their children get to school. Though there are many exceptions, Hutus tend to remain impoverished farmers. In Burundi the Tutsis have held on to power by suppressing the Hutus by force at every hint of an uprising. That domination came to end only last year when elections brought a Hutu to power for the first time. But in promising to restore land and create a racially balanced army, President Melchior Ndadaye threatened the ruling Tutsis and they killed him. In the Hutu uprising which followed, thousands of Tutsis were killed and the Tutsi army took revenge by killing thousands of Hutus. In Rwanda the Tutsis lost power at independence and many of them fled to Uganda. Here they lived like deposed aristocrats for 30 years but never forgot their homeland. Persecuted and scapegoated by successive Ugandan governments, their young men joined Yoweri Museveni's National Resistance Army in the early Eighties. The entire crowded fertile region of lakes and mountains in Central Africa has always been plagued by ethnic wars. The politics of the states have simply been a continuation of these wars, and until Museveni began a policy of giving all groups a stake in government, winner took all.

When Museveni's NRA took power in 1986 the Tutsis in his army formed an elite of officers, notable for their leadership and discipline. They began to plot their own return from exile. Today the RPF officer class is almost exclusively Tutsi, and most were born or raised in Uganda and fought for the NRA. In 1990 the signal was given and some 2,000 of them took their guns, stole what they could from the Uganda army stores and drove into Rwanda. They are a far more disciplined force than the government's, and are equipped by a cross-border operation from Uganda to which the Kampala government and the UN turn a blind eye. The weakness of the RPF is that despite having a chairman who is Hutu, it is about 90 per cent Tutsi. However decent its aims and disciplined its fighters, it is not perceived as a liberating force by Hutu people.

The RPF received some early setbacks but it soon held a substantial part of northern Rwanda. Peace negotiations bore fruit last year with the signing of a peace agreement, but President Juvenal Habyarimana never implemented it, playing a two-track policy of delay and arming the militias.

We will probably never know who shot down his plane on 6 April, but that was the signal the militias were waiting for. The massacres started immediately. Given the history of these former kingdoms and their neighbours, and the bitterness caused by the genocide of the last year, it is likely that the killings will spread. By August we may look back on this period as only the prelude.

(Photograph omitted)