Relatives of the abducted girls have received little information about the rescue effort
Police in Nigeria have appealed to parents of more than 200 abducted schoolgirls to come forward with photographs of the 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 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.
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.