This is the moment a heroic police officer leaps into a frozen lake to save an 11-year-old boy who had fallen through the ice in New Jersey.

Authorities reported that the child had wandered 68 metres across the frozen Franklin Lake before the ice suddenly cracked, dropping him into 4ft of icy water in northern New Jersey on Monday, February 3.

Police bodycam footage shows Officer Dave Brosonski breaking through the ice with his knees and rushing to the panicked youngster.

Brosonski said: 'He was yelling he couldn't feel his feet or arms. I just told him I'm on my way, everything's gonna be all right.'

Once pulled to safety, the boy was treated by first responders and taken to a hospital, where he is expected to make a full recovery.

Despite being hailed as a hero, Brosonski downplayed the praise: 'I feel it was my job. I would've gone in no matter what, for anyone, whether I was working or not.'

Officials have since warned residents against walking on frozen lakes, as recent warm weather has weakened the ice.

Police also emphasised how quickly hypothermia can set in, regardless of swimming ability.

Victims immersed in cold water can develop hypothermia, which occurs when the body loses heat faster than it can produce it.

Hypothermia is a medical emergency that occurs when the body's temperature drops below 95 degrees Fahrenheit (35 degrees Celsius). It can be fatal if left untreated.