Nigerian security forces on Saturday detained four U.S. citizens and a Nigerian who were travelling in the western Niger Delta, the scene of numerous militant attacks, a military commander said.

The group, which included the Nigerian head of a local non-government organisation working in the Delta, were detained in the Warri area while sailing in a boat along one of the swampy region's numerous creeks.

The Niger Delta is where the bulk of Nigeria's oil is produced and where local militants frequently attack oil installations and kidnap foreign nationals as part of a campaign to seek greater autonomy and a bigger share in the wealth of the world's eighth largest oil exporter.

Brigadier-General Wuyep Rintip said a military patrol stopped the group and detained them for questioning to confirm their identities and find out why they were in the Delta, which has become a security zone as a result of the militant activity.

"There is a need to know why they were in the creeks without a military escort and without prior knowledge of the authorities," Rintip told Reuters, saying the group had been sent to the Nigerian capital Abuja for questioning.

"Seeing four expatriates and one Nigerian in the creeks without an escort is a cause for concern," said Rintip, who is commander of the Nigerian military Joint Task Force for the western Delta. He said his forces had received information that local militants were planning a spate of further kidnappings of foreigners.

A spokesperson for the United States embassy in Nigeria told Reuters that the embassy was working with the Nigerian authorities to find out more about the incident.

"We have been informed that four U.S citizens and one Nigerian have been detained in Warri," the spokesperson added.

The names of the four Americans held were not immediately available but it was believed they could be members of a U.S. film crew working in the Delta with a local non-government organisation. The head of the NGO was Joel Bisina, who was the Nigerian held.

In the recent past, Nigerian authorities have detained a number of foreigners on different occasions in the Niger Delta, including an American expert on the region, Judy Asuni, who was later released.