In a statement released on the eve of the April 14, fourth anniversary of the kidnap of over 200 schoolgirls in Chibok, Borno State, the United Nations agency also said more than 1,000 children had been abducted and 1,400 schools destroyed by the terrorists in the region since 2013.
"The recent safe return of more than 100 of the Dapchi girls following the determined efforts of the Federal Government, should give all of us confidence that all hope is not lost".
On April 14, 2014 Boko Haram gunmen seize 276 girls aged 12 to 17 from the Government Girls Secondary School in the remote town of Chibok in Borno state.
In September 2015, Buhari raises the possibility of an exchange of Boko Haram prisoners for the girls.
Yakubu Nkeki, the head of the Chibok girls parents association, told AFP: "We are praying for every Nigerian who is in the custody of Boko Haram".
However, Buhari in a statement on Friday through his Senior Special Assistant (SSA) Media and Publicity, Garba Shehu, assured parents of the girls that "their daughters will never be forgotten or abandoned to their fate, despite four long years since they were taken away by terrorists".
The presidency has said that it is unaware of claims by a journalist, Ahmed Salkida, that only 15 out of the remaining Chibok girls in Boko Haram captivity are still alive.
The former miliary ruler has repeatedly claimed Boko Haram was virtually defeated but while there have been clear army gains, security threats remain.
One hundred and seven were returned in mid-March.
He urged the parents to keep their hopes alive on the return of their daughters.
Most recently, suspected Boko Haram fighters abducted 110 students from an girls' boarding school in the northeast town of Dapchi in the middle of the night on February 19.
Boko Haram has used kidnapping as a weapon of war during the conflict, seizing women and girls to act as sex slaves or suicide bombers, and men and boys to fight.
She said the voices of the captive Chibok girls "may not be heard in public, but they sound clearly in the hearts of their parents, community and well-meaning Nigerians".
"These repeated attacks against children in schools are unconscionable".
"Far more support must also be provided for past victims", she said, proposing a register for abducted people.
Some Dapchi parents were in Chibok on Saturday in a show of solidarity. But the region's deepening humanitarian crisis, mainly sparked by Boko Haram's years-long insurgency, remains one of the world's most severe.