Harrowing footage has captured the terrifying moment panic-stricken children fled a blazing school in Japan by climbing out of a window.

In the video circulating across social media, children can be seen scrambling out of a fourth-floor window as thick smoke consumed part of the building.

One eyewitness reported seeing several children drop onto the second-floor roof shortly afterwards.

The witness alerted emergency responders on the scene.

In the brief clip, the wail of emergency sirens can be heard as at least eight children edge along a narrow ledge of the roof in a desperate bid for safety.

Billowing clouds of dense, dark grey smoke poured from the school's windows.

In the background, another group of individuals — appearing to be pupils — can be seen assembling outside the building.

The witness, posting on X in Japanese, described the sight of children unable to access the evacuation route as utterly terrifying.

He noted that all the children and teachers who escaped via the window appeared to have been successfully evacuated.

One news outlet has reported that 300 people were evacuated and all pupils and staff are believed to be safe.

The cause of the blaze remains unclear at this stage.