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Nigeria is cracking down on "illegal aliens" from neighbouring countries whom the authorities call a "nuisance". Niger and Chad seem to have been singled out in the clampdown with thousands of their nationals packed into lorries and taken to the border.
People from Ghana, Togo, Cameroon and other West African countries have also been removed from at least eight major cities across the country.
Politicians are accused of importing "ghost voters" ahead of April polls.
"The truth is that it is because of the elections," an immigration official who did not want to be named told the BBC News website.
"You know people from Niger speak Hausa which is the language of northern Nigeria and they are Muslims. Because of this cultural affinity, it's easy for them to pretend to be Nigerians and vote in the elections," the source said.
But the official reason given for the deportations is that the illegal aliens constitute a "public nuisance and security risk".
Nigeria's Interior Minister Olu Adeniji told the BBC that "only vagabonds and those engaged in armed robbery and other illegal acts" were being targeted in the immigration sweep, but did not give any figures.
The Niger embassy in Abuja says it has received complaints about the deportations from its citizens in Abuja and other Nigerian cities.
"We don't know why the government of Nigeria is deporting citizens of Niger. We have not been given any explanations whatsoever," Dan Barma Aboubakar told the BBC News website.
Most of the "illegal aliens" are being taken to Jibia, a border community in Nigeria's north-western Katsina state.
No reason was given for the arrests. All they do is to simply ask your nationality and if you admit to being from Niger, they just order you to get on their van
Sanda Mamman Nigerien trader
Some Nigerians who were also mistaken for Nigeriens were arrested in the operation and taken to Nigeria's northern frontier and ordered to go back home to Niger.
Sanda Mamman, a Nigerien trader in the south-eastern Nigerian city of Enugu, told the BBC that 22 of his compatriots had been arrested in the clampdown.
"No reason was given for the arrests. All they do is to simply ask your nationality and if you admit to being from Niger, they just order you to get on their van," Mr Mamman said.
"If we have committed any crimes, we expect that we'd be told and charges pressed."
He said most of the people being deported had paid "all the relevant fees" and obtained "all the relevant stamps on our Ecowas [Economic Community of West African States] travel documents and residence permits".
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