Nigeria's military says it killed Shekau a year ago, and authorities said in September that they had killed an imposter posing as him in videos.
In the video recording obtained by Reuters on Saturday, the man's face is difficult to see as he is filmed from a distance.
"We have married them off and they are all in their husbands' houses," the man claiming to be Shekau says.
"The over 200 Chibok girls have converted to Islam, which they confess is the best religion. Either their parents accept this and convert too or they can die."
The majority of the kidnapped girls were Christians.
It was not possible to independently verify the video, but it was given to local journalists through the same channels that Boko Haram has used to distribute video tapes for the past three years, in what has become the militant group's sole means of communicating messages through the media.
It was also the classic style seen in the group's previous videos -- the purported leader is standing in semi-desert scrubland surrounded by fourteen masked gunmen with four military jeeps in the background. Two of the gunmen are holding up Boko Haram's al Qaeda-inspired black flag.
Verifying the authenticity of the video was further complicated by the fact that the group is made of several competing, and sometimes cooperating, factions with little in the way of a centralised command structure.
Whoever the figure in the video is, its release is likely to raise doubts about whether talks between a Boko Haram faction and the government in neighbouring Chad will secure the release of the girls, who were kidnapped from a secondary school in Chibok, northeast Nigeria, in April.
The man in the video denounced Danladi Ahmadu, the self-proclaimed representative of Boko Haram in Chad with whom the government has been talking. Nigerian authorities have said repeatedly they believe Ahmadu is a real Boko Haram commander and that he represents the faction holding the girls.
He also denied the existence of a ceasefire called by the government two weeks ago to help make the talks a success, which has had no apparent effect on the level of violence in the country.
"Who says we are dialoguing or discussing with anybody? Are you talking to yourselves? We don't know anybody by the name of Danladi. If we meet him now we will cut off his head," the man in the video says.
"All we are doing is slaughtering people with machetes and shooting people with guns ... War is what we want."
Officials at the presidency, whose office is conducting the talks, did not immediately respond to a request for comment.
The five-year-old campaign for an Islamic state by Boko Haram, which has killed thousands and whose name means "Western education is sinful", has become by far the biggest menace to the security of Africa's biggest economy and top oil producer.
Its fighters have attacked targets almost every day for weeks and last week seized control of the town of Mubi, the district town of Nigeria's defence chief Air Chief Marshal Alex Badeh. It was Badeh who announced the ceasefire.
They robbed banks, burned down houses and hoisted their black flag over the Emir's palace, killing dozens of people and forcing thousands to flee, witnesses in Mubi said.
On Saturday the fighting continued, with a security source saying as many as 30 Boko Haram fighters were killed in a battle in remote community called Sabon Gari, after insurgents stormed it on motorcycles and Nigerian troops fired back to protect it.
A car bomb thought to have been planted by Boko Haram killed at least 10 people at a crowded bus stop in Gombe on Friday morning, emergency services said.
The government has blamed the violence on Boko Haram's allied criminal networks that it cannot control, and on the various competing factions within the group.
The man in Saturday's video, who spoke in the northern Hausa language with occasional phrases in English, also said the group is holding a "white man", without giving details.
The only known hostage seized in the northeast is a German teacher kidnapped from a college in the northeastern city of Gombe in July by gunmen widely assumed to be linked to Boko Haram.
