Nigeria’s Senate said Tuesday it was investigating claims that the state oil company illegally took more than $1 billion from natural gas proceeds to fund fuel subsidy payments.

Despite being Africa’s largest crude producer, Nigeria relies on fuel imports to meet domestic demand because of its dilapidated refineries.

The imported petrol is subsidised by the government in an increasingly opaque scheme that has been described as a sprawling web of patronage and mismanagement.

In October, the Senate said it was probing spending by the National Nigerian Petroleum Corporation (NNPC) of some $3.5 billion in import subsidies which were not approved by parliament.

“An adhoc committee was set up to investigate the $3.5 billion NNPC subsidy use. The particular issue of $1.05 billion that Nigeria took from NLNG (Nigerian Liquefied Natural Gas company) came up,” said Senate Leader Ahmad Lawan in a statement posted to the Senate’s official Twitter account.

The NNPC has said in the past that high oil prices and fuel smuggling to bordering West African countries were driving up fuel subsidy payments.

But there are growing fears that the ballooning subsidy bill could be due to corruption.