A militant group in Nigeria calling itself the Niger Delta Avengers issued a warning Thursday to oil companies, saying their campaign has just begun.

"To international oil companies, this is just the beginning and you have not seen anything yet," spokesman Mudoch Agbinibo said in a statement. "We will make you suffer as you have made the people of the Niger Delta suffer over the years from environmental degradation and environment pollution."

The militant group last week took credit for knocking Chevron's operations in the Niger Delta offline. The group said the attacks came after issuing an ultimatum to the Nigerian government about developments in the Niger Delta.

While making no direct reference to militancy, Royal Dutch Shell halted regional operations early this week. The Shell subsidiary in Nigeria said in an emailed statement crude oil exports were interrupted Tuesday because of a leak on a regional pipeline.

According to the Organization of Petroleum Exporting Countries, member-state Nigeria produced an average 1.7 million barrels of oil per day in March, the last full month for which data are available. A country profile from the U.S. Energy Information Administration said Nigerian production peaked at 2.4 million bpd in 2005, and activity since then has been curbed in part by violence in the Niger Delta region that forced many energy companies to evacuate their staff.

The militant group said it was frustrated by what it saw as a lack of attention to the region paid by Nigerian President Muhammadu Buhari.

"We have been strategizing on how the voice of the Niger Deltans can be heard," Agbinibo said. "We are fighting a radical fight without taking any life."

The Nigerian government in March called for the division of the Nigerian National Petroleum Corp. into dozens of distinct entities in an effort to address corruption and revenue losses. In its country profile, the EIA said the lack of transparency, tensions over the division of oil revenue, and the "fragile situation" in the Niger Delta was problematic for oil-rich Nigeria.

The International Monetary Fund warned the impact of lower oil prices were adding to pressure from an economy in need of deep structural reforms.