The Government Science secondary school in Kankara district was attacked by armed gunmen on Friday.
Abuja, Nigeria CNN  — 

Some 337 students are still unaccounted for after gunmen attacked a school in northern Nigeria over the weekend, a government official told CNN on Monday.

Local police said a large number of attackers riding motorbikes ambushed the all-boys Government Science Secondary School in Kankara late Friday, in a possible kidnapping-for-ransom attempt.

The apparent kidnappers are making a demand via a teacher at school, Abdu Labaran, the director general of media for the governor of Nigeria’s Katsina state, said Monday.

“The abductors of the Kankara students have contacted a teacher and asked him to tell the government to stop the helicopter surveillance. They have not asked for ransom,” said Labaran, who added that the military had deployed in the area to search for the boys.

He said that at least 446 students had been handed over to their parents, but they have been unable to reach scores of families to confirm the safety of students because of bad phone connectivity.

The United Nations strongly condemned the abductions on Sunday, and called for the “immediate and unconditional release” of the children.

There have been varying estimates of the number of children that were kidnapped from the school late on Friday. Government officials said it was difficult to accurately track the numbers as some children ran away during the attack, while others escaped and made their way back to villages and the school over the weekend.

Garba Shehu, a spokesman for Nigerian President Muhammadu Buhari, told CNN via text message on Monday that some of the students had been interviewed by military officers.

“Some of the returning students debriefed by the military disclosed that ten of their colleagues were taken by the bandits, but this still needs to be verified,” he wrote, adding that there could still be others unaccounted for.

“The forests and neighboring villages are being searched and parents are being contacted for information about their children.”

A school official told CNN that at least three of his students escaped their abductors and returned to the school on Sunday. He said they had not eaten and that they had bruised and swollen legs from hours of trekking. CNN is not naming the school official to protect his safety.

“They commanded the crowd like a herdsman herd the sheep,” Hassan Abdul-Bashir, a 13-year-old student at the school, told CNN on Saturday. He said the gunmen were asking students for money, ransacking their lockers and taking some of their belongings. “They shot the policeman guarding our school. I saw them driving many students.”

Parents gather at the school on Sunday following the abductions.

Several more students were rescued on Monday, according to Labaran.

“Fifteen students were rescued today by military in their search and rescue operations, while the police rescued another person and a third came back to parents’ house himself,” he said Monday.

Musa Adamu, a senior secondary student at the school, said he and many of his roommates jumped out of the windows when they heard the gunshots on Friday. “We headed to the fence and climbed on it and jumped down,” the 18-year-old told CNN. “The gun shots sound got louder, we ran in different directions … into the forest. Most of us had no shoes on, and we kept running until we got tired and the sound of the gunshots fainted.”

Nigerian authorities said the motive for the attack is unclear, but added the area has seen kidnappings for ransom attacks in the past.

UN Secretary-General António Guterres urged the Nigerian authorities to “bring those responsible for this act to justice.”

“The Secretary-General reaffirms the solidarity and support of the United Nations to the government and people of Nigeria in their fight against terrorism, violent extremism and organized crime,” his spokesman, Stéphane Dujarric, said in a statement Sunday.