At least 700 million children - around one in four - have been robbed of their childhoods too soon by factors ranging from illness and conflict to child marriage and being out of school, according to a new report and index by Save the Children.

Most of these children live in disadvantaged communities in developing countries, where they have been bypassed by progress in health, education and technology that has improved the lives of many of their peers, the charity said in the report.

"Many of these children suffer from a toxic mix of poverty and discrimination, and experience several childhood enders," Save the Children International's Chief Executive Helle Thorning-Schmidt told the Thomson Reuters Foundation.

The hardest-hit children live in West and Central Africa - which accounted for seven of the 10 bottom-ranked countries in Save the Children's first ‘End of Childhood' index, ranking 172 nations according to where childhood is most intact or eroded.

Niger, Mali and Central African Republic were among the five worst-ranked countries for children in the annual index, which was topped by Norway, Slovenia and Finland.

While conflict and poverty are major threats to the futures of children, many also suffer because of discrimination, with girls, child refugees and those from ethnic or religious minorities among the most vulnerable, the charity said.

Countries must tackle discriminatory policies and practices, boost data and invest in public services if the world is to meet the U.N. global development goals set in 2015, said the report.

The 17 Sustainable Development Goals (SDG), which committed to end extreme poverty and inequality by 2030, pledged to reach first those who are most in need, and to leave no-one behind. "We have no chance of achieving the SDGs unless the basic right to childhood is protected," Thorning-Schmidt said.

Worldwide, half as many children under the age of five now die from preventable causes compared to 1990, while the number of those out of school or trapped in forced labor has declined by around one third since 2000, according to various studies.

Despite this progress, at least a quarter of the world's children are faced with one or more threats to their future, with early marriage and pregnancy linked to the risk of infant mortality and missing out on education, Save the Children said.

Violence is a growing concern with more and more children living in conflict, where they are twice as likely to die before five as those in peaceful nations, said Save the Children. In Central African Republic, Mali and Nigeria - countries beset by militant violence and mass displacement - more than one in 10 children die before their fifth birthday, compared with one in 500 in a country such as Finland, World Bank data shows. Yet these deaths are more often caused by disease, malnutrition and poor healthcare than by bombs or bullets.

Aid agencies must therefore think beyond traditional aid and integrate long-term development approaches and resilience-building efforts into their response to crises, experts say. "Temporary solutions aren't good enough anymore ... we need to realize people could be in crisis situations for years rather than months and work towards making them much more resilient," said U.N. children's agency (UNICEF) spokesman Patrick Rose.