Speaking in Hausa and Arabic, Shekau restates his claim of responsibility made in a video released last Monday and said the girls had converted to Islam.
"These girls, these girls you occupy yourselves with ... we have indeed liberated them. We have indeed liberated them. Do you know we have liberated them? These girls have become Muslims," he said.
The Boko Haram leader said that his brothers in arms had been held in prison for up to five years and suggested that the girls would be released if the fighters were freed.
"We will never release them (the girls) until after you release our brethren. Here I mean those girls who have not submitted (converted to Islam)," he added.
Abuja said late Monday that it would "use whatever kind of action" necessary to return the abducted children to their parents.
"At the moment, because all options are open, we are interacting with experts, military and intelligence experts from other parts of the world," Mike Omeri, the director of the government's information agency, said. "So these are part of the options that are available to us and many more."
Boko Haram has been engaged in a deadly insurgency in Muslim north since 2009, attacking schools, churches and government targets.
Civilians have borne the brunt of recent violence, with more than 1,500 killed this year alone while tens of thousands have been displaced after their homes and businesses were razed.
Nigeria's government has been criticized for its lack of immediate response to the kidnapping. President Goodluck Jonathan has now accepted help from the United States, Britain, France, China and Israel, which have sent specialist teams to help in the search effort.
French President Francois Hollande on Sunday offered to host a summit in Paris next Saturday with Nigeria and its neighbors focused on the armed group. On Monday it was reported that Jonathan had agreed to attend the meeting.
The leaders of Benin, Cameroon, Chad and Niger might also go and Britain, the European Union and the U.S. would probably be represented as well, Hollande's aides said.
Boko Haram has used kidnapping of women and young girls in the past and Shekau indicated that more were being held.
Eleven girls were abducted from the Gwoza area of Borno state on May 4.
Ros Jordan contributed reporting. Wire services