Nigerian security forces say they have detained four U.S. citizens and a Nigerian man for illegally traveling in the country's restive Niger Delta region.

Nigerian officials say the group was arrested on Saturday while traveling by boat along one of the Delta's numerous oil-rich creeks.

Security forces have classified much of the Delta as a military zone and barred foreigners from traveling there without the express consent of the authorities.

A U.S. Embassy spokesman told the Reuters News Agency that the embassy was working with Nigerian authorities to find out more about the incident.

Nigeria has detained a number of foreigners in the Delta in recent months as security forces have stepped up efforts to quell unrest in the region.

The bulk of Nigeria's oil is produced in the Delta. Local militants frequently attack oil installations there and kidnap foreign nationals in a campaign to secure access to a bigger share of the country's oil wealth.

Some information for this report was provided by AP and Reuters.