A teenage girl who managed to escape kidnapping by Islamist group Boko Haram during a night raid in Nigeria last month has spoken publicly about the ordeal, as international assistance to locate more than 200 girls still held hostage ramps up.

Militants stormed a secondary school in the village of Chibok, near the Cameroon border, on April 14 and abducted the girls, who were taking exams at the time.

In an exclusive interview with CNN, one of the girls, who asked not to be identified, said the kidnappers were loading the girls into seven lorries when she escaped.

"He said go and enter this car - a big lorry," she said.

"They say OK - enter this lorry, we go. I say I will drop down.

"We run in the bush. We ran and ran and we were gone.

"I feel afraid."

More than 200 girls are still being held hostage, and Boko Haram leader Abubakar Shekau says the group intends to sell the teenagers.

"I abducted your girls. I will sell them in the market, by Allah," he said in a video released shortly after the attack.

What is Boko Haram? Boko Haram, which means "Western education is sinful", is loosely modelled on the Taliban movement in Afghanistan.

Boko Haram, which means "Western education is sinful", is loosely modelled on the Taliban movement in Afghanistan. The group considers all who do not follow its strict ideology as infidels, whether they are Christian or Muslim.

The group considers all who do not follow its strict ideology as infidels, whether they are Christian or Muslim. It demands the adoption of Sharia law in all of Nigeria. Read more

"Allah has instructed me to sell them. They are his property and I will carry out his instructions."

The group's name means "Western education is sinful" and in the video, Shekau makes reference to the fact that the girls were undergoing Western education.

The abduction has sparked worldwide outrage, with several nations offering military and intelligence assistance to locate the girls.

Diplomats, aid workers, defence officials and other experts from Britain and the United States have been working with the Nigerian government since Saturday.

Nigerian president Goodluck Jonathan says he is optimistic about finding those still held hostage, citing international support, with Israel the latest country to offer help.

Israeli prime minister Benjamin Netanyahu did not give details of the proposed assistance but Mr Jonathan confirmed the offer to send a team of counter-terrorism experts was made during a phone conversation.

A viral social media campaign calling for the schoolgirls' return has seen public figures such as US first lady Michelle Obama tweeting with the hashtag #BringBackOurGirls.

On Sunday, French president Francois Hollande called on African leaders to hold a summit in Paris on security in west Africa.

The leaders of at least five African countries - Nigeria, Chad, Cameroon, Niger and Benin - may take part, a source close to the president said.

ABC/AFP