Two children have died of bird flu in Indonesia, the first deaths there from the virus in several months, hospital staff said Monday.

A 10-year-old boy and his five-year-old sister died at the Sanglah hospital on the resort island of Bali on Sunday, said Dr Ken Wirasandhi, a member of the hospital's bird flu unit.

"The patients' conditions drastically deteriorated" after they were admitted on Friday with high fever and tested positive for the H5N1 virus, he said.

"They came to us in a state of shock and despite our best efforts, they couldn't be saved," he added.

Officials say they are still investigating whether the siblings had come into direct contact with birds.

Indonesia is the country hardest hit by the virus. According to the World Health Organization website, as of March, H5N1 had killed 144 people in Indonesia since 2003.

The virus typically spreads from birds to humans through direct contact, but experts fear it could mutate into a form easily transmissible between humans, with the potential to kill millions in a pandemic.