Nigeria's militant Islamist group Boko Haram has abducted women and children in response to the arrest of its members' wives and children, it says.
Boko Haram leader Abubakar Shekau released a video, showing women and children purportedly being held.
If confirmed, these would be the first Nigerians taken hostage by Boko Haram.
In the video, Mr Shekau also said the group was behind two recent attacks in the north-east, which left an estimated 240 people dead.
The group has rejected an amnesty offered by the government to end the insurgency that has killed at least some 2,000 people in the past three years.
Boko Haram has repeatedly accused the security forces of illegally detaining the relatives of its members, saying their release was a pre-condition for any truce.
In the video, Mr Shekau did not name the women and children whom he said the group had seized, or say how many they were.
"We kidnapped some women and children, including teenage girls," he said.
This was in response to the securing forces arresting women, children and infants related to Boko Haram members in Kano, Bauchi and Damaturu, Mr Shekau said.
"In a single house in Damaturu, eight of our women and 14 children were arrested," he said.
Mr Shekau added that "no-one in this country will enjoy his women and children" if the relatives of Boko Haram members were not released.
Nigeria's government has not yet commented.
Mr Shekau confirmed the group had carried out a series of attacks in recent weeks - including a raid on 7 May by about 200 heavily-armed men on Bama village, in Borno state near Nigeria's north-eastern border with Cameroon.
"We are the ones that carried out the Bama attack," he said.
Five-five people were killed and 105 prisoners freed in the raids on a police station, military barracks and government buildings.
Mr Shekau said Boko Haram had also carried out a "small operation" on the northern town of Baga on 16 April.
Claims that Boko Haram members were killed in a shoot-out with security forces were "lies", he said.
"None of our people were killed in Baga," Mr Shekau added.
The army says it had killed 30 Boko Haram members in Baga, while one soldier and six civilians also died.
Rights groups accused the security of retaliating with excessive force, killing nearly 200 people and setting fire to many buildings in the town - an allegation the authorities have denied.
In April, Boko Haram released a French family of seven that were abducted in Cameroon in the first cross-border attack carried out by the group.
A confidential Nigerian government report, seen by Reuters news agency, said Boko Haram was paid more than $3m (£2m) to free the hostages.
Both France and Cameroon denied paying a ransom.
Boko Haram is fighting to create an Islamic state across northern Nigeria, where most people are Muslims.