Nigerian crude oil production in May was the lowest in more than a decade because of militant activity in the Niger Delta, a report from OPEC said.

A group calling itself the Niger Delta Avengers emerged in late spring to declare war against national and international oil companies operating in the Niger Delta. The group said it was frustrated that the government in Abuja is favoring the industry and the corresponding revenue stream over the welfare of the people living in the Niger Delta.

Vowing to bring some operators to a halt, the group last week said it bombed one of the major crude oil pipelines running through the region. A new group calling itself the Ultimate Warriors of the Niger Delta surfaced mid-June to say it would shut Chevron's operations down completely unless the government hands over 60 percent of the oil blocks in the Niger Delta.

Of the 13 members of the Organization of Petroleum Exporting Countries, Nigeria ranks seventh in terms of overall production. The country is one of the top exporters to the United States, the world's leading economy.

In a comment on short-term market disruptions in May, OPEC said "Nigerian output slumped to levels not seen in over a decade on the back of a wave of militant activity."

OPEC reports Nigeria crude oil production for May averaged 1.4 million barrels per day, down 15 percent from the previous month. Production averaged 1.8 million bpd during the fourth quarter of last year.

Nigeria relies heavily on oil for a source of revenue. In its latest survey, the International Monetary Fund said the challenges for Nigeria's economy are "substantial." Government deficit has doubled and total exports are down roughly 40 percent.

The IMF estimates the Nigeria economy will decline for the rest of the year, while inflation runs close to 10 percent.