This is the terrifying moment two boys were seen sitting on a balcony of an apartment block in Hong Kong.

Nail-biting footage shows the older five-year-old child relaxing on a concrete ledge with an air conditioning unit behind him, dangling his legs over the drop on March 21.

Above him, on the fourth storey, his three-year-old brother is sitting on a platform beneath the family's apartment balcony as police beckon him to scramble back inside.

Authorities said the boys had been left home alone by their parents and climbed onto the balcony through an unlocked window.

Horrified passersby called police, who smashed their way into the locked flat and staged a high-rise rescue.

Footage shows officers calling the younger boy in. He stands on the narrow ledge and grasps a uniformed officer's hand before being hauled to safety.

His older brother, however, refuses to budge. An emergency responder in climbing gear and a harness scrambles down and secures him with a safety line before the clip ends.

The children's parents, identified as Li, 32, and To, 29, were found nearby and arrested. Police said the pair face up to 10 years in jail for child neglect.