Nigeria's main militant group said one of its commanders and 62 of his followers were not captured by the military, but rather turned themselves in for a promised cash reward.
The military had said it arrested the militants Friday in southern Nigeria's Rivers State. On Saturday, it paraded the men in front of the media at an air force base in the southern oil hub of Port Harcourt.
In a statement Monday, the Movement for the Emancipation of the Niger Delta (MEND) refuted the government's account of the arrests, saying its fighters "handed themselves over" with "no exchange of gunfire."
Nigerian authorities say the militants kidnapped 19 oil and construction workers in the Niger Delta region. The hostages, who were freed last week, included seven foreigners.
The group has threatened more attacks on oil facilities.
Some information for this report was provided by AFP and Reuters.