JOHANNESBURG (AFP) - South African police arrested a boy for hacking to death four family members with an axe after two siblings escaped and called for help, a spokesman said on Sunday.

The 14 year-old was arrested shortly after the grisly murder of his mother, grandmother, brother and baby sister on Friday morning, said police spokesman Lungelo Dlamini.

"It's alleged that he used an axe and then attacked them while they were sleeping," Dlamini told AFP.

The boy's eight-year-old and nine-year-old sisters ran away during the killings at 1am (7am on Friday Singapore time) in the Etwatwa informal settlement east of Johannesburg.

"The sisters informed the neighbours what was happening at the house and in the mean time the boy ran away," Dlamini told AFP.

Police tracked down the teen by midday.

The mother, 42, was found dead in the corrugated iron-shack the family lives in, while the three-month-old sister was found dead in a nearby field.

The granny, 58, and brother, 7, died in hospital.

Police are still searching for the murder weapon.

The boy will appear in the Benoni Magistrates court on Monday.

South Africa has one of the highest murder rates outside war zones.

Just under 16,000 people were killed between 2011 and 2012, according to official statistics.