By Adaobi Tricia Nwaubani and Kieran Guilbert for the Thomson Reuters Foundation.

Nigeria is facing mounting pressure to find some 200 schoolgirls abducted 1,000 days ago in Boko Haram's most infamous attack after the rescue of 24 girls raised hopes that they are alive.

For more than two years there was no sign of the girls who were kidnapped by the Islamist fighters from a school in Chibok in northeast Nigeria one night in April 2014, sparking global outrage and a celebrity-backed campaign #bringbackourgirls.

But the discovery of one of the girls with a baby last May fuelled hopes for their safety, with a further two girls found in later months and a group of 21 released in October in a deal brokered by Switzerland and the International Red Cross.

Hear one rescued Chibok's story of what life was like under Boko Harem.

For parents like Rebecca Joseph the return home of the group of 21 girls at Christmas was a bitter-sweet celebration.

Her daughter, Elizabeth, is one of an estimated 195 girls still held captive by the jihadist group, which has tried to force some of them to convert to Islam and to marry their captors.

"I am happy that some of the girls are returning home even though my own daughter is not among them," Joseph told the Thomson Reuters Foundation in the town of Chibok in Borno state.

"My prayer is that my daughter and the rest of the girls will be rescued and returned to their families safe."

With last weekend marking 1,000 days since the girls were abducted, President Muhammadu Buhari said he remained committed to ensuring the abducted schoolgirls are reunited with their families "as soon as practicable".

"We are hopeful that many more will still return," said Buhari, who came to power in 2015 and replaced a government criticised for not doing enough to find the missing girls.

"The tears never dry, the ache is in our hearts," he said in a statement.

The Nigerian government said last month that it was involved in negotiations aimed at securing the release of some of the girls as the army captured a key Boko Haram camp, the militant group's last enclave in the vast Sambisa forest.

The exact number of Chibok girls still in captivity is believed to be 195 but it has been hard to pin down an exact number since the girls went missing.

Academics and security experts say it may be a huge challenge to obtain the girls' freedom given the significance of the abduction for Boko Haram, which has killed about 15,000 people in its seven-year insurgency to set up an Islamic state.

"Outside Nigeria, the Chibok girls have come to symbolise the Boko Haram conflict," said Sola Tayo, an associate fellow at the London-based think tank Chatham House.

"The global outrage generated by their captivity has added to their value to the insurgents," she added, adding that they were also significant to Buhari because he made their release a key campaign pledge before his 2015 election.

The government said in October that it had not swapped Boko Haram fighters or paid a ransom for the release of the 21 girls but several security analysts said it was implausible that the Islamist group would have let the girls go for nothing.

"To secure the release of the remaining girls would require concessions by the Nigerian government, which could reverse significant gains it has made against Boko Haram," said Ryan Cummings, director of risk management consultancy Signal Risk.

"In addition to detainees, Boko Haram may also demand supplies, weapons, vehicles and even money which they could use to recalibrate and invigorate their armed campaign against the Nigerian state."

Deep Divisions

One of the major obstacles to securing the release of all of the Chibok girls who remain in captivity is the deep divisions emerging within Boko Haram, said Freedom Onuoha, a security analyst and lecturer at the University of Nigeria in Nsukka.

The militants split last year with one faction moving away from the group's established figurehead Abubakar Shekau over his failure to adhere to guidance from Islamic State to which Boko Haram pledged allegiance in 2015.

It is unclear how many Chibok girls are held by the main faction led by Shekau, thought to be based in the Sambisa, and by the Islamic State-allied splinter group - headed by Abu Musab al-Barnawi and believed to operate in the Lake Chad area.

"It will be difficult to release most of the remaining girls as each faction will maintain a strong hold on them and would negotiate with state officials on their own terms," said Onuoha.

While the deal to free the 21 girls was seen as a huge boost for the government's assertions that it would soon bring home the others, a lack of progress since then has seen public hopes dwindle and frustrations arise, academics said.

Although Nigeria has driven Boko Haram out of most of the territory it held, its battle against the militants will not be considered over until the fate of all of the Chibok girls is made clear, said Nnamdi Obasi of the International Crisis Group.

"From various indications, it is most unlikely that all the remaining girls will come home alive, but the government owes their parents and the public the fundamental responsibility of accounting for every one of them," the Nigeria analyst said.

"In the long run, that's the only way to bring closure to this sad episode."

(Reporting by Adaobi Tricia Nwaubani and Kieran Guilbert, Editing by Belinda Goldsmith. The Thomson Reuters Foundation, the charitable arm of Thomson Reuters, covers humanitarian news, women's rights, trafficking, property rights, climate change and resilience. Visit http://news.trust.org)