The Nigerian government has rejected an offer from Islamist extremists who have kidnapped more than 200 schoolgirls in the country.

Boko Haram offered to release them if some of its members who are in prison were also freed. But the Nigerian Interior Minister has said the group would not be allowed to 'give conditions'.

It comes as a new video issued by the group claims to show some of the schoolgirls, who the group's leader says have converted to Islam.

AFP reported that Boko Haram's leader said the girls would not be released until Boko Haram prisoners are freed.

Police say a total of 276 girls were abducted on April 14 from the northeastern town of Chibok, in Borno state, which has a sizeable Christian community. Some 223 are still missing.

In the video, Abubakar Shekau speaks for 17 minutes before showing what he says are about 130 of the girls, wearing full-length hijabs and praying in an undisclosed rural location.

One of the girls then appears to talk directly to the camera.

None of the youngsters appears to be visibly distressed, but it appears as if they are clearly under duress.

Holding a pad of paper in his hand, Mr. Shekau tells the camera "These girls, these girls you occupy yourselves with their affair we have indeed 'liberated' them. We have indeed 'liberated' them".

"Do you know 'we have liberated them'? These girls have become Muslims. They are Muslims".

He continues "It is now four years or five years that you arrested our brethren and they are still in your prison".

"You are doing many things (to them). And now you are talking about these girls. We will never release them until after you release our brethren".

Reporter Alex Crawford who has spoken to a father of one of the kidnapped girls says he does not want the government to release Boko Haram prisoners in exchange for his daughter.

He told her "Its not right. They'll do it again".

Meanwhile the girls' parents have been expressing their anger that school staff did nothing to stop their abduction.

A special adviser to the country's president Dr. Reuben Abati said there were lines the government would not cross in the hunt for the girls.

Speaking after it was revealed authorities had made indirect contact with Boko Haram, Dr. Abati said "The government of Nigeria has no intention to pay a ransom or to buy the girls, because the sale of human beings is a crime against humanity".

"The determination of the government is to get the girls and to ensure that the impunity that has brought this about is checked and punished".

Boko Haram's leader Abubakar Shekau has threatened to sell the girls "at the market" and some are believed to have already been taken out of the country.

The search for the girls remains centred on the huge Sambisa forest, which is three-times the size of Wales.

France said that Nigerian President Goodluck Jonathan had agreed to attend a summit in Paris on Saturday to discuss what to do about Boko Haram.

The EU, the United States, Britain and the four countries bordering Nigeria have all been invited: Benin, Cameroon, Chad and Niger.

Several of the countries in the region affected by the consequences of Boko Haram violence are French speaking.