The government of Niger and French nuclear energy group Areva announced on Monday that they had signed a deal to renew a decades-old agreement for the operation of two uranium mines.

Under the deal, negotiated for 18 months, Areva agreed that a 2006 mining law sharply increasing taxes on mineral extracted would apply to the Somair and Cominak operations in the north of the country which it partially controls.

"We have heard the government's legitimate call for higher receipts coming from uranium," said Luc Oursel, Areva CEO, on a visit to Niamey to sign the deal.

However, a joint statement said that the operations would be exempt from sales tax over the course of the five-year deal.

The revenue issue had been the main sticking point in the talks since the government considered that the previous contracts, which expired at the end of last year, were unfavourable to the country, the fourth-biggest producer of uranium in the world.

The French arm of charity Oxfam, which has been a sharp critic of state-controlled Areva's uranium dealings with Niger, said the new deal continued to shortchange Nigeriens, who stood to lose "10 to 15 million euros a year." ($13.6 to $20.5 million)

Uranium is crucial to providing electricity to France, a country that is roughly 75 percent powered by nuclear plants. About 20 percent of France's energy comes from Niger's uranium.

The partnership between French state operators and the Niger government dates back to the colonial era and has often been criticised as being unfair to Africans. Areva sales worldwide in 2012 were more than four times Niger's national budget.

Areva employs about 6,000 workers in Niger and the deal also commits the French firm to building a new headquarters in Niamey, as well as a highway to the uranium belt in the north of the country.