Lagos, Nigeria(CNN) The parents of kidnapped Nigerian schoolgirl Leah Sharibu have told CNN that terror group Boko Haram has issued an execution deadline for their daughter.

Nathan Sharibu said Boko Haram militants threatened to kill their daughter in October if their demands were not met in a video shown to him by local media reporters.

Her parents have issued desperate pleas to the Nigerian government to continue negotiations with their daughter's abductors as it has done for other schoolgirls abducted by the militants in February.

"The terrorist threatened to kill Leah in October if they don't get any response to their demands. Time is running out, that is why I am calling on the government to keep talking to them," her father Nathan said.

"Our daughter is facing a death sentence," he added.

Presidential spokesman Garba Shehu told CNN Thursday that the government was working with international organizations, which helped in past negotiations to plead with the abductors following the ultimatum.

"The President contacted international negotiators when they issued the threat some few weeks ago. We are prevailing on them to spare Leah's life and that of others in their captivity, " Shehu said.

President Buhari has also called the family directly to assure them that his administration is doing all it can to secure her release.

Rebecca says the family has been in turmoil since the group issued the ultimatum.

"The militants were angry that they had not gotten any response. I am praying and people are praying with us, but we are now in October, I can't sleep knowing my daughter's life is in danger," she said.

Sharibu, 15, was abducted alongside more than 100 schoolgirls by a faction of Boko Haram from their boarding school in Dapchi village, Yobe State in February.

Most of the students were were released after four weeks. However, Sharibu was held back after she reportedly refused to renounce her Christian faith.

An audio of the teenager pleading for her freedom was released by her abductors in August.

Boko Haram has kidnapped more than 1,000 children in Nigeria since 2013.

The group, however, gained global notoriety following the mass kidnapping of more than 200 girls from a boarding school in Chibok town, Borno State in April 2014.

More than 100 Chibok girls were freed in a swap deal between the Nigerian government and the insurgent group over the past two year but around 100 of them remain missing.