The Dangote Oil Refinery has called on Nigeria’s upstream oil regulator to force producers to abide by a law that stipulates they supply local refineries, saying that lax enforcement was raising its operational costs.

The 650,000-barrel-per-day capacity refinery, built by Africa’s richest man Aliko Dangote on the outskirts of Lagos for $20 billion, has struggled to get sufficient supplies from Nigeria, where vandalism and low investment impede oil production.

In a statement issued on Friday, Dangote Refinery accused the Nigerian Upstream Petroleum Regulatory Commission (NUPRC) of failing to enforce the Domestic Crude Supply Obligation (DCSO), a provision that requires crude oil producers to supply domestic refiners with a portion of their production.

“Our concern has always been that the NUPRC is pushing, but the international oil companies are not following the instructions,” said Anthony Chiejina, a Dangote Refinery spokesperson in the statement.