Police in Nigeria have appealed to parents of more than 200 abducted schoolgirls to come forward with photographs of their daughters.
The girls were taken from their school in Borno state by suspected Islamist militants more than two weeks ago.
Borno state's police chief told the BBC that the authorities needed to confirm exactly who was missing as the school records had been burned in the attack.
He said it was now thought that 223 girls were still missing.
The Islamist group Boko Haram has not made any response to the accusation that its fighters abducted the girls from the school in Chibok town in the middle of the night on 14 April 2014.
The group, whose name means "Western education is forbidden" in the local Hausa language, has staged a wave of attacks in northern Nigeria in recent years, with an estimated 1,500 killed in the violence and subsequent security crackdown this year alone.
'Girls from several schools'
Tanko Lawan, Borno state's police commissioner, said the headmistress of the school in Chibok had been working to produce a list of those believed to have been taking their final year exams.
Her task had been hampered as students from surrounding areas had also come to the school to take the exams as it was believed the town was relatively safe from attack.
He said current figures showed that 53 of the girls were believed to have escaped.
But he added that it was difficult to know for sure, as some parents may not have informed the authorities if their daughters had returned home.
"That's why we're appealing to parents to come with their photographs so that we know actually [that] these are the numbers we are dealing with," he told the BBC Hausa service.
Since the kidnapping, the number of missing girls has been disputed and parents have criticised the government's search and rescue efforts.
Earlier this week, a community leader in Chibok said that 230 girls were missing - a significantly higher figure than officials had been quoting - and 43 had escaped.
This week protests have been held across Nigeria, calling on the government to do more to help secure their release.
It is thought that the militants initially took the girls to the Sambisa forest; there have been subsequent reports they have been taken over the borders into Chad and Cameroon and possibly forced to "marry" the insurgents.
Swathes of north-eastern Nigeria are, in effect, off limits to the army, allowing the militants to move the girls with impunity, says the BBC's Will Ross in Abuja.
A security source told Nigeria's Vanguard newspaper that four army battalions have been deployed to the area and an offensive on the forest was planned.
Former British Prime Minister Gordon Brown has called for international military assistance to be offered to Nigeria in the hunt for the girls.
"We could provide military help to the Nigerians to track down the whereabouts of the girls before they're dispersed throughout Africa - like air support, for example, if that was thought necessary," he told the UK's Guardian newspaper.
Last week, an advisor to Nigeria's president said the government would welcome international assistance.
Boko Haram leader Abubakar Shekau first threatened to treat captured women and girls as slaves in a video released in May 2013.
It fuelled concern at the time that the group was adhering to the ancient Islamic belief that women captured during war are slaves, with whom their "masters" can have sex, correspondents say.