The death toll from the devastating blaze at Endarasha Hillside Academy school in Nyeri County has risen to 21, the government said on Saturday.

Prosecutors said they had instructed police to look into whether the tragedy had been caused by negligence or recklessness, and vowed that those found culpable would face justice.

The flames engulfed a dormitory at the primary school in the county of Nyeri around midnight on Thursday as more than 150 boys aged between nine and 13 were sleeping.

Government spokesman Isaac Mwaura said a total of 19 bodies had been recovered at the site and another two died in hospital.

Out of the total 156 boys in the dorm at the time, 139 had now been accounted for, either at home or in hospital, he said.

"It is a catastrophe beyond our imagination," Mwaura said at a press briefing at the site.

"It is truly devastating for the nation to lose such a number of young and promising Kenyans. Our hearts are heavy."

The cause of the inferno is not yet known but homicide investigators and forensic experts are at the school, where media access has been blocked.

The charred bodies of victims, which police had said were burnt beyond recognition, were found in the dormitory, now a blackened shell with its corrugated iron roof completely collapsed.

Chief government pathologist Johansen Oduor said postmortems will begin on Tuesday.

Director of Public Prosecutions Renson Ingonga has instructed police to conduct a comprehensive investigation to determine the circumstances that led to the fire and "assess whether or not the tragedy may have resulted from negligence and/or recklessness of any responsibility holders," his office said in a statement.

"Any person found culpable towards the fire tragedy shall be expediently taken through the due process of a criminal trial."