Nigeria's main militant group says it has freed 19 Nigerian oil workers kidnapped by other gunmen nearly a month ago.

The Movement for the Emancipation of the Niger Delta, or MEND, says in a statement that it released the hostages Sunday in Nigeria's Rivers state.

MEND said it rescued the hostages in mid-September, a few days after they were abducted by local pirates.

The group said today it is still holding a Ukrainian and two British nationals captured with the other hostages.  It says they cannot be released because of "security concerns."

The oil-producing Niger Delta has been the scene of violent unrest during the last three years, much of it focused against the oil industry.  

Militants say they want impoverished local residents to get more of the region's oil wealth.

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