(CNN) -- Nigerian government officials and the International Committee of the Red Cross have had talks with Boko Haram about swapping imprisoned members of the Islamist terrorist 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 release 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 terrorist 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 location, 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.
"ICRC couldn't find where the remaining 24 were being detained," the source said.
The six prisoners included Kabiru Sokoto, a senior Boko Haram commander convicted in December 2013 of terrorism charges related to the fatal Christmas Day bombing of a church in the town of Madallah in 2011.
The Boko Haram negotiators said they would get back to the government after consulting with superiors.
ICRC sources declined to comment.
The name "Boko Haram" translates to "Western education is sin" in the local Hausa language. The militant group is trying to impose strict Sharia law across Nigeria, the most populous country in Africa.
In recent years, its attacks have intensified in an apparent show of defiance amid the nation's military onslaught. Its ambitions appear to have expanded to the destruction of the Nigerian government.
Ongoing militant attacks
Dozens of merchants were killed this week when Boko Haram gunmen launched a rocket and gun attack on a local market in northeastern Borno state, witnesses said Saturday.
The attack happened Thursday but details were slow to emerge because of a mobile phone shutdown in the region Friday. Phone signals were restored only on Saturday, residents said.
Scores of Boko Haram militiamen wearing military and police uniforms stormed a market in the town of Mainok, 31 miles (56 km) outside Maiduguri, during peak hours, the witnesses said.
The militants fired rocket-propelled grenades and fired on traders. They looted food and loaded it onto trucks abandoned by fleeing traders, witnesses said.
"They killed dozens of people in the raid but it is difficult to give precise toll because everybody fled the market to escape the gunmen," said local merchant Babagana Kyari, who witnessed the attack.
The attack was an attempt by the militia to replenish depleted food supplies in areas under its control, said Madu Kachalla, another trader at the market. "They seized cash from traders and loaded food stuff into trucks they impounded at the market before fleeing into the bush," Kachalla said.
Boko Haram fighters looted a truck carrying sacks of grain to Maiduguri on Wednesday before setting it on fire, residents said.
CNN's Ray Sanchez contributed to this story.