More children are surviving past the age of five worldwide, but it's not enough, the U.N. Inter-agency Group for Child Mortality Estimation (UN-IGME) said on Thursday.

While the child mortality rate has dropped from 12 million in 1990 to 6.9 million in 2011, there is still much left to achieve the U.N.'s Millennium Development Goal of cutting child mortality by two-thirds by 2015.

"Proven solutions need to be expanded to accelerate progress on child survival faster and farther," the group said in a statement.

The UN-IGME, which consists of UNICEF, the WHO, the World Bank, and the U.N. Department of Development and Social Affairs, said that the leading causes of death of children under five are pneumonia, premature birth complications, complications during birth, diarrhea and malaria.

It also said that only six of the 10 designated world regions were on track to meet the development goal, and that in 2011, 19,000 children still died every day from preventable causes. Eighty percent of those deaths occurred in Southeast Asia and Sub-Saharan Africa.

One reason for the decrease in deaths is tied to the increase in wealth worldwide. The UNICEF statement emphasized that "neither a country's regional affiliation nor economic status need be a barrier to reducing child deaths," and pointed out that several low-, middle- and high-income countries had succeeded in slashing their child mortality rates by two-thirds.

But the connection between growing health and growing wealth is hard to deny in this case, and it has been studied.

UNICEF Executive Director Andrew Lake said: "Millions of children under five are still dying from largely preventable causes for which there are proven, affordable intervention."

The report also noted that "infectious disease are characteristically diseases of inequity, disproportionately affecting poor populations."

Of the countries that the UN-IGME deemed on track for meeting the child mortality goal, several of them had seen significant growth in wealth and GDP since 1990, including Mexico, China, Peru and Brazil.

All four of these countries went from having under-five mortality rates of between 140-240 per thousand live births in 1990, to rates of 17 to 19 deaths per thousand live births, respectively, in 2011.

In 1990, Mexico's GDP was around $260 billion, China's was $360 billion, and Brazil's was $460 billion. As of 2011, Mexico's GDP jumped to $1.1 trillion, Brazil's reached $2.5 trillion, and China's skyrocketed to $7.3 trillion. Even Peru, the poorest of the four, saw its GDP increase six-fold, from $26.3 billion to $176.7 billion.

The countries with the highest rate of deaths per live birth, Somalia, Mali and Burkina Faso, also saw some GDP growth, and the under-five death rates dropped in Mali and Burkina Faso (Somalia stayed flat at 180 per 1,000). These countries remain among the poorest in the world, with CIA Worldbook GDP rankings of 191, 180, and 169 out of 194 countries, respectively.

"These lives could be saved with vaccines, adequate nutrition and basic medical and maternal care," Lake continued.

 "The world has the technology and know-how to do so. The challenge is to make these available to every child."