"I don't have any reports to the contrary. I would if it had changed significantly," Ajumogobia told Reuters when asked if output was still around 1.6 million bpd, excluding condensate, the level produced before the latest fighting began last week.

Nigeria last week launched its biggest military offensive for years in the western Niger Delta, bombarding militant camps from the air and sea before sending in hundreds of ground troops to try to flush rebel fighters out of local communities.

Some foreign producers have evacuated non-essential personnel from around the town of Warri for fear that militant fighters will launch retaliatory attacks on Africa's biggest oil and gas industry.

Nigerian oil production peaked at around 2.4 million bpd before the militant Movement for the Emancipation of the Niger Delta (Mend) burst on to the scene in early 2006 and knocked out almost a quarter of output in a matter of weeks.

Production has never fully recovered. Persistent pipeline bombings and kidnappings of foreign oil workers by Mend over the past three years have made it difficult for the industry to repair damaged infrastructure.

Ajumogobia told Reuters on Monday that at one point last month, production had dropped to between 1.2 million bpd and 1.3 million bpd, suggesting Nigeria has been struggling to meet its implied Opec quota of 1.67 million bpd, which came into effect on 1 January.

"It is something we are all sad about. Nigeria has a production capacity of 3.2 million bpd, today we are down to about half of that," Ajumogobia told reporters in the capital Abuja.

He said onshore production had been the worst affected in recent years, citing the case of Shell's facilities in the eastern part of the Niger Delta.

"Shell has lost most of its production in the east ... The irony of it is that onshore is the cheapest to produce and therefore the return on that investment is greater, but that is where we have most of the shut-in," he said.

Published: 21 May 2009 11:09 GMT  | Last updated: 21 May 2009 11:09 GMT