Rich countries should make more vaccine available to poorer nations where the H1N1 virus is starting to hit, U.N. health officials said on Sunday.

They said increased readiness for swine flu was needed in developing countries with weaker medical systems and with large, young populations, who are most vulnerable to the disease.

"We may well see a different pattern of impact once this virus starts to take off and those explosive outbreaks occur in some of the poorer communities in the world," said Julie Hall, an infections disease expert at the World Health Organization, a U.N. agency.

The WHO, which declared H1N1 a global pandemic in June, says a third of the world's population of nearly 7 billion people could catch it.

Some countries, such as the United States, Brazil and France, have agreed to make 10 percent of their national vaccine stockpile available to developing countries. Manufacturers have also donated about 150 million doses of vaccine.

More is needed, said David Nabarro, the U.N. coordinator for fighting new emerging flu varieties.

"The challenge during the next few weeks is to build up the solidarity between wealthy nations and poor nations to ensure that adequate vaccine is made available," he said.

September and October are usually just the start of flu season in the northern hemisphere, but there are signs of a second H1N1 wave after it first flared this year, Hall said.

"Already we are seeing the U.S., many European countries, Japan and Mexico reporting over the past few weeks a sudden increase in cases," she said.

As of September 20, swine flu had killed 3,917 people in 191 countries since being identified in April, the WHO has said.