Mike Ryan, co-ordinator of the WHO's global outbreak alert and response network, said: "At this point, our global emergency stockpile is depleted.

"We are very much on the edge of our ability to supply both emergency campaigns and these preventative mass campaigns.

"It is a rather uncomfortable position for us to be in."

William Perea, the WHO's yellow fever chief, said yellow fever can "spread like a fire in the forest," adding that mosquitoes thrive especially in urban areas with poor hygiene and sanitation.

Perea said many of Latin America's cities have not been exposed to the virus and therefore have not developed any immunity.

In the jungle, where most yellow fever outbreaks occur, people have better defences against the virus.

One million doses

Antonio Barrios, Paraguay's vice public health minister, said: "At any moment more than one million doses will arrive from France.

"Asuncion is not in any danger of infection," he said.

Authorities said they previously received about one million doses donated by Brazil, Peru, Venezuela and other sources.

But Barrios acknowledged that about 75 per cent of those stocks were used up.

The WHO relies upon three pre-qualified manufacturers of yellow fever vaccine: France's Sanofi-Aventis, Senegal's Institut Pasteur, and Bio-Manguinhos in Brazil.

The current global production capacity is 30-35 million doses, Ryan said.

Ryan said it was important for the vaccines, which cost 60 US cents each, to be replenished quickly and maintained at healthy levels.

"We do need to ensure better security in the number of manufacturers we have and the scale of production available," he said.

Yellow fever is named after the jaundice that affects some of those infected with the viral haemorrhagic disease.

The WHO estimates that 200,000 people catch yellow fever each year, and 30,000 die as a result.