Indonesia launched a major bird flu drill Friday that will test the ability of the country hardest hit by the virus to respond to a possible pandemic. Thousands were taking part, from local residents to government officials.

The three-day simulation started with the isolation of a village on the resort island of Bali, where a field hospital was being set up to treat people with flu-like symptoms. At Sunday's closing, officials will try to prevent "infected" travellers from leaving the international airport and spreading the virus to other countries.

"This is a very important event from the perspective of public health," said Subhash Salunke, of the World Health Organization. "It will certainly help better equip Indonesia in the event of a pandemic. But other countries struggling to contain bird flu outbreaks can and will learn from this exercise as well."

Indonesia has been worst affected by bird flu since it started ravaging Asian poultry stocks in late 2003, with its 107 human deaths accounting for nearly half the 240 recorded fatalities worldwide. The government has come under fire for failing to slow the spread of the virus.

Others, like Vietnam, have succeeded thanks largely to strong political will, mass culling of all chickens in infected areas and aggressive vaccination campaigns. But Indonesia says there are limits to what its cash-strapped government can do.

The virus remains hard for people to catch, but scientists worry it could mutate into a form that spreads more easily between humans, with the potential to kill millions worldwide. Indonesia is seen as a potential hotspot for that to happen.

That makes the drill on Bali especially relevant, said Nyoman Kandun, a senior Health Ministry official. More than 5,000 people were taking part, from government officials and law enforcement officers to doctors and villagers.

"We want to show the world that we are prepared, that we are ready to contain and stop this virus in the event of a pandemic," he said.