ABUJA (Reuters) - Nigeria will close the capital city of Abuja's airport at the end of Wednesday for six weeks of scheduled runway repairs, an aviation ministry spokesman said, after airlines threatened to stop flying there.

Passengers will be diverted to Kaduna airport, about 160 km (100 miles) north of the capital. They will then be transported on guarded buses to Abuja, along a road where kidnappings have taken place in recent years.

Kaduna airport has primarily been used for domestic flights, with Abuja airport handling 4,859 domestic flights in December compared with the 171 that flew in or out of Kaduna.

The government, which announced plans for the shutdown in December, hopes international carriers will use Kaduna as an alternative during the closure. But on Monday workers were still needing to fit electrics, seating and toilets in the new terminal there.

Airlines including British Airways, Lufthansa and South African Airways have refused to fly into Kaduna. Ethiopian Airlines has said it will use the alternative airport.

Aviation ministry spokesman James Odaudu confirmed the closure would begin Wednesday after some confusion over when the shutdown would start.

On Monday, Nigeria's aviation minister said the country will open up its government-owned airports to private investment, in a bid to improve their operation.

(Reporting by Felix Onuah and Paul Carsten; editing by Julia Glover)