The United States ranks 39th in the world when it comes to children 'flourishing', falling behind countries including Saudi Arabia and Belarus, a new United Nations backed report says.

America also comes in at the eighth worst nation for sustainability when excess carbon emissions are taken into consideration, beaten only by countries including Trinidad and Tobago and Qatar.

The new global index showed children in Norway, South Korea and the Netherlands had the best chance at survival and well-being thanks to good healthcare, education and nutrition.

But it showed the US ranked lower than 38 other nations when it comes to the same indicators.

The ranking of countries by per-capita carbon emissions put those and other rich nations, including the US and Australia, close to the bottom on that measure, as major contributors to global health threats driven by climate change.

'Countries need to overhaul their approach to child and adolescent health, to ensure that we not only look after our children today but protect the world they will inherit in the future,' said former New Zealand Prime Minister Helen Clark, co-chair of the international commission that produced the report.

The United States ranks 39th in the world when it comes to children 'flourishing', falling behind countries including Saudi Arabia and Belarus, a new United Nations backed report says

The ranking of countries by per-capita carbon emissions put those and other rich nations, including the US and Australia, close to the bottom on that measure, as major contributors to global health threats driven by climate change

The 'flourishing' part of the index measures countries on the geometric mean of surviving and thriving.

Top ten countries include the United Kingdom, Ireland and Japan.

The report states: 'The USA is a higher income country (HIC) that ranks as the 11th most unequal country in the world (among countries for which we have data on income inequality). Moreover, the child flourishing rank of the USA (39th) is also poor compared with many other HICs, and even some middle income countries (MICs).'

The 'sustainability' part of the index ranks countries on how their per-person emissions compare with a 2030 target giving a two-thirds chance of keeping global average temperature rise to 1.5 degrees Celsius above pre-industrial times.

Of the top 25 countries with the best score on emissions, all but two were African.

That contrasts starkly with the 'flourishing' part of the index, where many African nations did badly on children's health, education, nutritious food and protection from violence.

Not one country performed well on all three measures of child flourishing, sustainability and equity, concluded the commission convened by the World Heath Organization, The Lancet medical journal and U.N. children's agency UNICEF.

Another key threat identified was exploitative marketing practices that push fast food, sugary drinks, alcohol and tobacco at children, increasingly through social media channels.

The report said dramatic progress had been made in improving children's lives in the past five decades but economic inequalities meant the benefits were not shared by all.

And the heating up of the planet and damage to the environment, among other stresses, meant every child faced an uncertain future, it added.

Not one country performed well on all three measures of child flourishing, sustainability and equity, concluded the commission convened by the World Heath Organization, The Lancet medical journal and U.N. children's agency UNICEF

'Climate disruption is creating extreme risks from rising sea levels, extreme weather events, water and food insecurity, heat stress, emerging infectious diseases, and large-scale population migration,' said the report by more than 40 experts.

Commission member Sunita Narain, director general of the New Delhi-based Centre for Science and Environment, said children's health today 'is at grave risk because of environmental degradation'.

'The biggest inequity that we need to confront today is the inequity (of) climate change,' Narain said.