Fourteen nationals from Britain, Russia, Ukraine and Japan were charged on Wednesday with illegally trading and storing petroleum products in oil-rich but cash-poor Nigeria.

Crude stolen from sabotaged pipelines and illegally refined oil products are regularly smuggled out and sold on the lucrative black market, an illicit business that is estimated to cost the country some much-needed $6bn a year in revenue.

Nigeria's anti-graft agency the Economic and Financial Crimes Commission (EFCC) arraigned the accused in a Lagos High Court for conspiring to store 4,500 tons of petroleum products in three vessels in February without authorisation.

It was not immediately clear if the products were to be taken out of Nigeria and if so, to which country they were headed.

The suspects pleaded not guilty to the charges with their lawyers urging the court to release them on bail.

Presiding judge Ibrahim Buba remanded the suspects in custody until they met their bail conditions.

They were also ordered to surrender their passports to the EFCC, while the court fixed another hearing for June 17.

Nigeria is Africa's leading oil producer but has no functioning refineries, forcing crude to be exported then its products imported in a process that has been seen as wide open to fraud.

The government keeps the cost of fuel at the pumps low, currently $0.44 per litre and pays fuel importers the difference between the market rate and their costs.

They shut depots several weeks ago claiming non-payment by the government, causing a crippling shortage of fuel that brought the country to a virtual standstill.

Agreement was reached to lift the blockade just days before Muhammadu Buhari was sworn in as president on May 29.

Buhari won elections two months earlier on a pledge to crack down on rampant corruption, including in the oil sector.