Nigerian security officials are investigating four Americans and a Nigerian man accompanying them on their trip to the troubled oil-rich delta region. The group was flown to the capital, Abuja, for breaking a travel ban on outsiders. Gilbert da Costa has the latest from the Nigerian capital.

A spokesman for Nigeria's Joint Task Force that is deployed in the Niger Delta, Major Omale Ochagwuba, told VOA the presence of the Americans in the delta without authorization is considered a serious security breach.

"They were seen by our troops going to the creeks without informing us of their presence, and without telling us of their mission," said Major Ochagwuba. "Ordinarily, if you report to a place, you should report to the unit on ground, to provide escort for you and any assistance you require. We should know their whereabouts; we should know their mission so we can give them protection."

The U.S. embassy says it is working with the Nigerian authorities to resolve the incident. The Americans are believed to be a U.S. film crew working in the Niger Delta with local groups.

The group was arrested while traveling in a boat along one of the swampy region's numerous creeks. The Americans were in the company of a Nigerian head of a local non-governmental organization working in the Delta.

The security forces are waging an offensive in the vast wetland region where the bulk of Nigeria's oil is produced and where militants have repeatedly attacked oil facilities and personnel.

The military has barred outsiders from traveling there without express consent by security forces. Udeng Eradiri, who speaks for the Ijaw Youth Council, a Niger Delta pressure group, says the Nigerian military is behaving like an occupation army and dismissed the travel ban as absurd and unacceptable.

"They caught them with a Niger Deltan, do they need clearance? Are they saying we should start taking clearance to navigate our rivers? If they were caught alone that would have been a different thing," said Udeng Eradiri. "My fiend is coming to visit me, somebody I am taking to my house and you tell me I should come and take clearance. That is rubbish. That is an insult to the Ijaw man. That we should now begin to seek clearance before people will come to our region. It is not acceptable and we will never accept it."

Security forces have detained a number of foreigners in the delta in recent months as the military steps up combat operations in the region.