Nineteen Nigerian oil workers who were kidnapped two weeks ago have been set free, the main militant group in the oil-producing Niger Delta area says.
But the Movement for the Emancipation of the Niger Delta (Mend) said it was still keeping two British workers and a Ukrainian who were also seized.
Mend said it had rescued the group of 22 from "pirates" who had originally taken them hostage.
Unrest in the Niger Delta has led to a 20% cut in Nigeria's oil production,
On Saturday, President Umaru Yar'Adua said the situation in the Niger Delta was a "nightmare", which was scaring away potential investors in the country.
Mend said it could not free the three foreigners because of "security concerns - due to the location where the expatriates were kept for their safety".
Another Briton was released on Saturday after more than two weeks in captivity.
More than 200 foreigners have been kidnapped by militants in the Niger Delta, the heart of the country's oil industry, since 2006.
Almost all have been released, normally after the payment of a ransom, although this is always officially denied.
Mend caused severe disruption by attacking gas plants, oil installations and pipelines. It claims to be fighting for greater control over oil wealth in the impoverished region.
But opponents say the militants make money from criminal rackets and trade in stolen oil.