Africa's biggest diesel and aviation fuel refinery has started production, the company said on Saturday, calling it a "big day for Nigeria" after years of construction delays at the plant.

Built by the continent's richest man, the 650,000 barrel-per-day (bpd) Dangote refinery could be a game changer when fully operational by helping end Nigeria's reliance on fuel imports.

The delayed megaproject built by Nigerian billionaire Aliko Dangote said it hoped the products would be on the market this month, but it was not clear when the refinery will reach full capacity production or start refining petrol.

The facility sits on 2,635 hectares (6,500 acres) of land at the Lekki Free Zone on the edge of Lagos City and cost an estimated $19 billion, according to local media.

"Dangote Petroleum Refinery has commenced production of diesel and aviation fuel," the group said. "This is a big day for Nigeria. We are delighted to have reached this significant milestone."

Although Nigeria is Africa's top energy producer, it has relied on imports for most of the fuel it consumes because of a lack of refining capacity.

The Dangote refinery is expected to not only make it self-sufficient but also allow it to export fuel to neighboring West African countries, potentially transforming oil trading in the Atlantic Basin.

Company officials told Reuters test runs could begin this week after the refinery received a sixth crude oil cargo on Jan. 8.

Nigeria swaps crude worth billions of dollars for petrol that it had subsidized for years to keep prices cheap for its domestic market.

Fuel imports and subsidies caused a huge drain on foreign exchange when Nigeria struggled with dwindling oil revenues and foreign currency shortages.

The refinery, first scheduled to open in 2021, was officially inaugurated by then-President Muhammadu Buhari earlier this year and was supposed to begin operations in June.

Since coming to office in May last year, President Bola Ahmed Tinubu has ended the long-standing fuel subsidy and floated the naira currency in economic reforms he says will attract foreign investment and build long-term growth.