Local governor Umaru Yacuba told the BBC that it was implementing in earnest a controversial decision to expel the Mahamid thought to number some 150,000.

The government ordered all the Arabs to go back to Chad after accusing them of wrongdoing, including theft and rape.

But an official backtracked, saying it will expel only those without papers.

Government spokesman Mohamed Ben Omar told the BBC it was an immigration issue and he estimated that only about 3,300 would have to leave.

When asked what the government would do about those who do not have the right papers, but who work as civil servants and in the police - or even as MPs, Mr Omar said they would have to think about it.

NIGER'S MAHAMID ARABS Originally nomads from Chad 150,000 live mainly in Diffa State Many came after 1974 drought More fled 1980s Chad fighting Fought against 1990s Tuareg rebellion

But in Diffa, security forces have started to round up groups of Arabs, escorting them to the border, houses and scrubland are also reportedly being searched.

On Wednesday, Mahamid leaders told reporters they would defend themselves against attack and called on the United Nations to intervene.

They insisted they were citizens of Niger and "have no other country to go to", after being given five days to leave the country.

Many of the Arabs came to Niger from neighbouring Chad following the 1974 drought in Chad.

Others who were fleeing fighting in Chad arrived in the 1980s. Many have since risen to senior positions in the military, local administration and in business.

Arid zone

Earlier this week, the governor of Diffa State, where most of the Mahamid live, told them it was "high time" to pack and return to Chad.

"We have decided, starting today, to expel these nomadic Arab 'Mohamides' to their home countries," Niger's Interior Minister Mounkaila Modi told national television.

"These foreigners have shown no respect to the rights of the natives and they're putting pressure on pastures in this region. We can no longer accept seeing our ecosystem degraded by foreigners."

Mr Modi said the Mahamid possessed illegal firearms and were a serious threat to the security of local communities and that their camels were draining local oases, Reuters news agency reports.

Like the rest of the country, the east of Niger is extremely arid.

It is populated by nomadic cattle herders, whilst the Arabs also own camels. Not surprisingly, one source of the tension between the communities is water.

With the Sahara desert expanding quite quickly there are growing fears that the scarcity of water could spark future problems in many African countries in the region.

The BBC's West Africa correspondent Will Ross says that with the spread of Islam to Africa in the 7th and 8th centuries, Arabs greatly expanded their presence and influence and there are many examples of how the African and Arab cultures have mixed.

For example, some 20% of East Africa's Swahili language comes from Arabic. Arab and non-Arab Africans both had the common goal of opposing European colonialists.

But there have also been areas where the cultures have clashed, one example being Sudan, which has been plagued by conflict between the Arab dominated government in the north and the black African south.