Nigerian government officials and the International Committee of the Red Cross (ICRC) have had talks with Boko Haram about swapping prisoners of the Islamist terror group for the more than 200 Chibok school girls kidnapped in April, a source involved in the negotiations told CNN.

The officials met four times in mid-August with two senior members of Boko Haram in Nigeria's capital, Abuja.

The swap would involve the released of 30 Boko Haram commanders in the custody of the Nigerian government, according to the source, who asked not to be named due to the sensitivity of the issue.

Boko Haram submitted a list with the names of 30 members who were either convicted or awaiting trial on terror offenses.

“The two Boko Haram negotiators assured the ICRC and government negotiators that the girls were never raped, were never used as sex slaves and were never sexually assaulted,” said the source, who attended the discussions.

The terror group abducted an estimated 276 girls in April from a boarding school in Chibok in northeastern Nigeria. Dozens escaped, but more than 200 are still missing.

Boko Haram has expressed a willingness for a swap with the ICRC at an undisclosed, according to the source. But there was disagreement on some terms, including the number of girls involved in the swap.

Boko Haram had insisted on an even swap – 30 girls for the 30 commanders – but the government refused, according to the source.

“They were only ready to release one to one, which the government was not going to accept,” the source said.

Another hurdle in the talks was Boko Haram’s insistence on meeting the imprisoned 30 members involved in the swap, but they only had contact with six at a prison outside Abuja, the source said.

The six prisoners included Kabiru Sokoto, a senior Boko Haram commander convicted in December 2013 of terror charges related to the deadly Christmas Day bombing of a church in the town of Madallah in 2011.

“ICRC couldn’t find where the remaining 24 were being detained,” the source said.

The Boko Haram negotiators said they would get back to the government after consulting with superiors.

ICRC sources declined to comment.

•Excerpted from a CNN report. Photo shows some of the abducted Chibok schoolgirls as appeared in a Boko Haram video.