For six weeks starting on Wednesday night, flights will be diverted to Kaduna while Abuja airport is closed for runway repairs after airlines threatened to stop flying there.

The cutting of direct flights to Abuja, an important business hub as well as Nigeria's political nerve center, raises economic and security concerns. Passengers will take the journey of around 160 km (100 miles) to the capital on guarded buses, along a road where kidnappings have taken place in recent years.

A Reuters team saw workmen on scaffolding putting electrical fittings in place and fitting tiles in some parts of the Kaduna complex.

Electric cables and unused scaffolding lay outside the building and parts of the complex remained unpainted, but inside there was electricity to power the conveyor belts.

Despite the ongoing work, a flight carrying passengers on Ethiopian Airlines, the only international airline that has said it will use the alternative airport, arrived at the new terminal at 11.30 local time (1030 GMT), said a senior airport official.

"Airport staff were very cooperative but I think the systems are not ready. The systems should be ready," said one passenger.

Bismarck Rewane, a leading Nigerian economist, has said the closure of the airport in Abuja, where the government and central bank are based, will have a "catastrophic" impact on Africa's biggest economy.

It comes amid a downturn in the OPEC member which, reeling from low oil prices and militant attacks on oil facilities in its Niger Delta energy hub, saw its economy shrink 1.5 percent in 2016 - its first full-year contraction in 25 years.

Airlines including British Airways, Lufthansa and South African Airways have refused to fly into Kaduna due to security concerns.

The Reuters journalists saw police vehicles stationed at about 2 km intervals on the road between Abuja and Kaduna.

Kaduna airport has primarily been used for domestic flights, with Abuja airport handling 4,859 domestic flights in December 2015 - the last month for which figures were available - compared with the 171 that flew in or out of Kaduna.