International oil companies operating in Nigeria owe $4 billion to an agency responsible for fostering development in the Niger Delta, the impoverished wetlands region where much of the industry is located, a minister said on Thursday.

Tayo Alasoadura, minister of state for Niger Delta Affairs, said the debt to the Niger Delta Development Commission (NDDC) had been accumulating over a long period of time, but did not name specific companies.

“International oil companies are expected to pay 3% of their annual budget to NDDC as their major income but they have been defaulting for a long period of time,” Alasoadura told reporters.

“Efforts are being made to get the outstanding payments which is up to $4 billion from them. All of them are owing,” he said.

It was not clear what potential enforcement mechanisms might be available to the NDDC or the government.

Asked to respond to Alasoadura’s remarks, a spokesman for U.S. major ExxonMobil said: “ExxonMobil complies with all governmental laws, rules and regulations applicable to its operations within and outside the United States.”

Spokespersons for Anglo-Dutch firm Shell and for Italy’s Eni said they would send responses later, while Chevron did not immediately respond to a Reuters request for comment.

Reuters