Nairobi/Abuja - Nigerian oil militants Sunday said they had released 19 Nigerian oil workers kidnapped two weeks ago but said they would hold on to two Britons and a Ukrainian taken at the same time.

The Movement for the Emancipation of the Niger Delta (MEND), which seized the workers from the group that originally kidnapped them, said in a statement that the expatriates could not yet be released due to "security concerns."

Militant groups in the Niger Delta often attack oil installations and kidnap expatriate workers, saying they are fighting for a greater share of profits from oil exploitation for the poor of the region.

The government says they are merely criminal gangs intent on stealing oil and extorting money.

Expatriate workers are normally released after money changes hands, although ransom payments are officially denied.

Another Briton, David Melford, was released in the oil-rich Niger Delta on Saturday.

The unrest had cut oil production by around a fifth since early 2006, helping to push up global oil prices and allowing Angola to surpass Nigeria as Africa's biggest oil exporter.