A well-known US aid worker and her two German companions have been arrested in Nigeria's oil-rich Niger Delta region.
Dr Judith Burdin Asuni has lived in the region for years
Security officials say Judith Burdin Asuni, Florian Alexander Opitz and Andy Lehmann are being held on suspicion of "espionage and terrorism".
The Germans had been filming masked youths from the Ijaw community in Delta State, allegedly without clearance.
Delta militants have been conducting a violent campaign for the oil-rich area to get a larger share of the oil money.
The BBC's Alex Last in Lagos says the two German nationals had come to Nigeria to do a preliminary research for a possible TV documentary about the Niger Delta.
Our correspondent says the Niger Delta is a sore subject for the Nigerian authorities, particularly the international attention given to militant groups.
The US embassy in Abuja told the BBC that it was in touch with the Nigerian government over the continued detention of Mrs Asuni, who is married to a Nigerian and has lived in the region for 36 years.
"All we know is that Judith Asuni is a peace worker who got funding from academics and international donor agencies to work for peace in Nigeria," the embassy said.
Mrs Asuni runs a high-profile non-governmental organisation called Academic Associates Peace Work and has run workshops with the Nigerian police on conflict management.
The Germans were detained last week by members of the State Security Service (SSS) after filming the masked youths.
A few days later, the SSS also arrested Mrs Asuni whose organisation had provided some assistance to the two Germans.
According to a security service official who spoke to the BBC, they suspected the three of an attempt to carry out an act of terrorism.
"The lady is suspected of espionage by exploiting the situation in the Niger Delta," Addo Mwazu said.
Other reports suggest the arrests were because their actions intended to embarrass Nigeria.
The Niger Delta is home to all of Nigeria's oil, responsible for 95% of hard currency earnings, but most of the peoples of the Delta live in abject poverty.
This week, the main militant group in the region, the Movement for the Emancipation of the Niger Delta, called off a truce with the Nigerian government after the arrest of one of its leaders in Angola.
It had announced a voluntary truce after the May inauguration of President Umaru Yar'Adua, but says talks have failed.
Last week, President Yar'Adua ordered an investigation into alleged links between government officials in the Niger Delta and violent criminal gangs.