Nigeria's anti-corruption police arrested six Ghanaians and a Nigerian for stealing millions of dollars worth of crude oil in the restive Niger Delta, the agency's spokesman said on Sunday.

The suspects were taken into custody after their vessel, carrying a cargo worth around 330 million naira ($2.2 million), was seized in Chanomi Creek in southern Nigeria's Delta state in late December.

"The vessel was loaded with 4,000 metric tonnes of petroleum product suspected to be crude oil," said Femi Babafemi, spokesman for the Economic and Financial Crimes Commission.

Nigeria and its partners, including international oil firms such as Royal Dutch Shell and Chevron, lose a sizeable chunk of revenues each year to oil theft, known locally as "bunkering".

Some estimates put the amount of crude stolen from the region at 100,000 barrels per day, equivalent to around $4.6 million daily or $1.7 billion a year at today's prices.

Bunkering is part of a network of highly organised crime which can often involve collusion between mafia-like gangs, local government officials, international shipping agents and members of the armed forces, security experts say.

Thieves drill into pipelines or hijack barges loaded with oil, often using militant attacks as cover, before shipping the spoils out of Nigeria to be sold on the international market.