Nigeria has called off a deal with Islamist group Boko Haram for the release of some of the abducted schoolgirls.
Some of the girls were set to be freed in exchange for imprisoned Islamist militants.
Boko Haram group snatched more than 200 girls from a school on April 14.
Nigeria’s government is under pressure to do more to tackle the group and bring about the girls’ release.
Thousands of people have died since Boko Haram began a violent campaign against the Nigerian government in 2009 and in the subsequent security crackdown.
Officials have held talks with the group to secure the release of the schoolgirls.
An intermediary met Boko Haram leaders earlier this month and visited the location in north-east Nigeria where the girls were being held.
A deal was almost reached to set some of the girls free in exchange for the release of 100 Boko Haram members being held in detention.
But the government cancelled the planned agreement shortly before the swap was due to take place.
The reasons for the withdrawal are unclear.
It came just after Nigerian President Goodluck Jonathan attended a meeting in Paris hosted by President Francois Hollande of France where leaders said they had agreed a “global and regional action plan” against Boko Haram.
The girls, who were mainly Christian, were taken from their school in Chibok, in north-eastern Borno state and are thought to be held in a remote forested area of the state, close to the border with Chad and Cameroon.
Nigeria previously insisted it would not agree to free Boko Haram members in return for their release.
The UK, the US, China and France are among the countries to have sent teams of experts and equipment to help to locate them.