GENEVA, May 15 (Reuters) - Average life expectancy has risen globally to 73 years for a girl born in 2012 and 68 for a boy following successes in fighting diseases and child mortality, the World Health Organization said on Thursday.

Big advances in the battles against infectious diseases such as measles, malaria, tuberculosis and polio have continued to extend life expectancy although other factors, such as people's lifestyles, are constraining longevity, the WHO said in its annual statistics report.

The longest life expectancy at birth is for baby girls in Japan, at 87.0 years, and boys in Iceland, at 81.2 years. Japan, Switzerland, Singapore, Italy and Luxembourg rank in the top 10 for both sexes.

"There are major gains in life expectancy in recent decades and they continue," said Ties Boerma, chief of statistics and information systems at the WHO.

The lowest life expectancy is in sub-Saharan Africa, where nine countries have expectancy of less than 55 for babies of both sexes.

Lifestyle changes leading to heart problems and other diseases were curbing life expectancy in some cases.

"We're seeing a health transition from success in infectious diseases to more people dying, including at younger ages, from non-communicable diseases," said Boerma.

However, even in the rich countries where people live longest, there is no sign of life expectancy gains slowing down.

"If human life expectancy was capped at population level at around 90 years of life we would expect to see a slowdown as we approach those limits. We're not seeing that," said Colin Mathers, coordinator of WHO's statistics on mortality.

For the first time the annual statistics report, the most comprehensive statistical overview of the world's health, measured "years of life lost", a number that takes account of the age when people die as well as the number of deaths, to put more focus on the things that kill more people at a younger age.

Years of life lost to diarrhoea and respiratory infections, the biggest causes of early deaths in 2000, had fallen by 40 percent and 30 percent respectively by 2012, when ischaemic heart disease was the biggest factor in early deaths.

Years of life lost to road injuries have also increased by 14 percent between 2000 and 2012 as more people driving in developing countries outweighed gains in road safety elsewhere.

CARDIOVASCULAR DISEASE

Life expectancy at birth has increased in almost every country since 1990, and in almost all cases it was higher in 2012 than in 2011, with Botswana, Cote d'Ivoire and Syria among the exceptions.

Another was Pakistan, where life expectancy averaged 65 years in 2012, down from 67 years in last year's report. Mathers said that reduction reflected improved data, which revealed child mortality was 30 percent higher than previously thought.

The global average life expectancy for a 60-year-old has also increased, with hope of another 20 years in 2012 instead of 18 years in 1990. But not everywhere.

In Russia and several other former Soviet states, life expectancy for a 60-year-old was lower in 2012 than in 1990. Mathers said death rates rose fast in the early 1990s, due to cardiovascular disease and injury, both influenced by high rates of binge drinking. Although the death rates have since improved, they have not yet dropped to pre-1990 levels.

Russia's experience surprised epidemiologists who had expected chronic diseases would take a long time to take root.

Many countries, especially those recovering from conflict, have also shown that it is possible to make big gains fast.

"We've seen in many countries a catch-up, really fast progress," said Boerma. "Examples - Liberia, which has been our fastest catch-up country, but also Rwanda, Cambodia. So if they come out of a crisis with good leadership, there's enormous progress in health."