PRESIDENT Umaru Yar'Adua has ordered Nigeria's state-run oil company to demand nearly $2bn in arrears from two major oil companies.
The government says Royal Dutch Shell and ExxonMobil have not paid taxes and production sharing costs they owe on two offshore oil fields.
The companies however said that they have followed the law and were in discussions with the government.
Militant attacks and funding troubles have seen Nigeria cut its production.
"ExxonMobil affiliate fully complies with all laws and regulations and has paid taxes and royalties to Nigeria accordingly," AFP reported ExxonMobil responding.
"Our affiliate routinely has ongoing discussions with a number of government agencies on a variety of topics, including tax.
"We do not comment on ongoing discussions," it said.
Shell Nigeria Exploration and Production Company (Snepco) refused to comment while there were "ongoing discussions" between it and the government.
