"The epidemiological investigation concluded so far can rule out the possibility of human-to-human transmission (of the disease)," said Director of the Animal Disease and Health Control Department Nyoman Kandun at a press conference.

However, further investigation is needed to be done around such possibility, especially after the international community cautioned against the biggest cluster of bird flu death found in Indonesia's North Sumatra Province recently.

Indonesia, assisted by two epidemiologists from the World Health Organization's (WHO) headquarters and an independent laboratory in Atlanta, the United States, has been conducting investigation on the possible human-to-human transmission of bird flu, he said.

"We do not know a lot about avian influenza. So the strategy is research and development. The global community will focus their eyes on Indonesia and then they will help us," he said.

On the same occasion, WHO's Technical Officer Steven Bjorg told Xinhua that it is very urgent to determine whether the human-to-human transmission has taken place in Indonesia.

However, "it was very difficult to prove that," he said.

On the case in North Sumatra where six people died of bird flu, Kandun told Xinhua that no evidence of human-to-human transmission has been found, but the virus is supposed to have the longest incubation period and was not detected in the first examination.

Some 27 out of 33 provinces in Indonesia have been contracted with bird flu, while human fatality stands at 32 and infections at 43.

All over the world, the WHO has raised the confirmed human death toll from the H5N1 bird flu strain to 122, while the total number of confirmed human infections since the current outbreak began in 2003 has reached 216.

Experts fear that the virus can mutate into a certain form that easily transmits from human to human, which will kill millions of people.