Four of the eight people from a North Sumatra family have died of bird flu, according to local tests, while one person who has tested positive is undergoing treatment. Of the other three, two have died of suspected bird flu, but there is no confirmation from local tests yet. The last person is alive and is being tested for bird flu.

"So far, an examination of chickens, ducks and pigs around the victims' houses has turned out negative," Apriyantono told reporters on the sidelines of a Food and Agriculture Organisation meeting in Jakarta. "But we still have to thoroughly investigate it as it's a complicated case."

Experts have in the past said that cluster cases or possible bird flu cases among family members do not mean the virus is mutating but could be caused by close contact normal in families. A health ministry spokeswoman said blood samples of the five people who had tested positive locally had been sent to a World Health Organisation-affiliated laboratory in Hong Kong for confirmation. Locals tests are not considered definitive.

The World Health Organisation has also sent a team to North Sumatra to investigate the case since last week, said Sari Setiogi, a spokeswoman for the agency in Jakarta. The WHO has confirmed 25 fatalities from avian influenza in the world's fourth most populous nation, the second highest number of human deaths after Vietnam.

The virus has spread in birds at an alarming rate in recent months, sweeping through parts of Europe, down into Africa and across into South Asia. It is difficult for humans to catch, but experts fear the virus could evolve into a form passed easily from human to human, causing a pandemic that could kill millions.

In Indonesia, the H5N1 virus has been reported in birds in about two-thirds of the country's provinces. But Indonesia's government has resisted mass culling of birds, citing the expense and impracticality in a country where keeping a few chickens or ducks in the backyards is common.

ThePoultrySite News Desk