International donors pledged $4.3 billion to vaccinate nearly 250 million children against life-threatening diseases such as pneumonia and diarrhoea, at a London conference Monday.

GAVI (Global Alliance for Vaccines and Immunisation), the group backed by Microsoft tycoon Bill Gates, said the funding would save more than four million lives over the next four years.

Britain pledged £814 million ($1.3 billion, 920 million euros) in additional funding and Gates said his charitable foundation would add another $1 billion over five years.

The pledges surpassed the $3.7 billion (5.3 billion euro) target set by GAVI.

"We have exceeded the figure that we set ourselves and we have received firm pledges for a sum of $4.3 billion," Britain's International Development Secretary Andrew Mitchell announced.

Among other donors, Norway gave $677 million and the United States pledged $450 million, while Brazil and Japan donated to GAVI for the first time.

Brazil said it would give $12 million from 2011 to 2015 and Japan pledged $9 million.

British Prime Minister David Cameron told the conference: "Frankly, the idea of children dying from pneumonia and diarrhoea should be absolutely unthinkable in 2011.

"But for many parents in the developing world it is a devastating reality."

GAVI is aiming to immunise 243 million more children by 2015.

It has already vaccinated 288 million children in 19 countries and now wants to extend the programme of jabs to another 26 countries.

Pneumonia and diarrhoea kill three times as many children under the age of five as HIV/AIDS even though vaccines are available to prevent such deaths.

Many developing countries cannot afford the vaccines.

British drug giant GlaxoSmithKline last week announced it would slash 95 percent off the price of a vaccine for the diarrhoeal disease rotavirus for sales to the world's poorest nations.

Mitchell said it was now possible to vaccinate a child "for the price of a cup of coffee".

Gates told AFP in an interview: "The great thing is that as these vaccine prices continue to come down, that should free up funds for new vaccines and (we can) spend money to spread coverage."

He said around 20 percent of children in at-risk countries were currently missing out on being vaccinated.

"The kids that are missed are the ones most at risk. We have to fund a cold chain, keeping the vaccines cold otherwise they spoil.

"We have to communicate with the mothers so they know they have to demand the vaccines for their children and they know it's a safe programme. So money is going to go for that communications effort."

However, medical charity Doctors Without Borders said some of the vaccines GAVI bought were overpriced and claimed there was a "conflict of interest" because pharmaceutical group Johnson and Johnson is part of the GAVI board.

Gates however said the vaccines were "often less than a tenth what the United States or United Kingdom pay to buy the same vaccine."

Liberian President Ellen Johnson-Sirleaf hailed the conference as "an important moment in our collective commitment to protecting children in developing countries from disease", but warned against complacency.

"Every 20 seconds, a child still dies of a vaccine-preventable disease. There's more work to be done."

U2 singer Bono, who founded the ONE campaign against poverty and disease, said that diarrhoea could be "death sentence" in Africa.

"Vaccines are a global health game-changer," he said.

"The goal of saving four million kids in the next few years is within reach.

"Vaccines are simple, powerful, cost-effective tools that can save millions of lives. It's crazy just how much sense it makes."