Image caption The greatest risk is during the first few days after birth, the report says

Child mortality has fallen by more than 50% since 1990, a report by the World Health Organization and UN children's agency Unicef says.

It says that 25 years ago 12.7 million children under five died, but this year the figure is projected to drop below six million for the first time.

But aid agencies warn that huge challenges remain.

They point out that the UN target of reducing child mortality by two-thirds between 1990 and 2015 will not be met.

The rate fell by 53% over this period, the report says.

Stark inequality

"We have to acknowledge tremendous global progress," said Unicef's deputy executive director Geeta Rao Gupta.

"But the far too large number of children still dying from preventable causes before their fifth birthday... should impel us to redouble our efforts to do what we know needs to be done".

Lowest and highest rates of child deaths

Luxemburg and Nordic countries including as Iceland , Finland and Norway are among those with the lowest deaths among under fives with less than three per 1,000 births

and Nordic countries including as , and are among those with the lowest deaths among under fives with less than three per 1,000 births Oil-rich Angola has the highest rate of child deaths up to 254 per 1,000 births, followed by Somalia, Chad and Central African Republic

* Figures from World Health Organization Child Mortality Report based on the upper bound figure for deaths of children under five per 1,000 live births in 2015

The report says that 16,000 children under the age of five still die every day. Many become victims of preventable illnesses such as pneumonia, diarrhoea or malaria.

And almost half the deaths are linked to malnutrition, the document says.

The greatest risk is during the first few days after birth - 45% of all deaths occur before the child is a month old.

The report also highlights the stark inequality of life chances for the world's children.

It says that those born in sub-Saharan Africa have a 1-in-12 chance of dying before their fifth birthday. In wealthy nations the risk is 1-in-147.