Some 21 others have suffered severe symptoms including nausea, stomach pains, dizziness and diarrhoea, they said.
Two women aged 40 and 65 were the latest victims and died on Friday, said Yuliani, from a hospital treating the patients who are from Central Java's Kanigoro village in Ngablak district.
"We still don't know what kind of illness they suffered," she told AFP.
A team of experts from Indonesia's health ministry and Yogyakarta's Gadjah Mada university were at the hospital investigating the case, she said.
Two patients remained in a critical condition while 19 were improving, with nine of them being treated at a small community clinic close to their village, she said.
Marwan Nusri from the health ministry's disease control department said that autopsies carried out on the first eight victims showed they had suffered liver dysfunction, but what caused it was unclear.
"We are still investigating the cause... The preliminary results show that it might be a virus, fungal toxins or heavy metal pollutants which have contaminated water," he said.
Nusri said that the victims did not suffer any respiratory difficulties, meaning that neither bird flu -- which has killed some 81 people in Indonesia -- nor Severe Acute Respiratory Syndrome, was to blame.
Severe Acute Respiratory Syndrome (SARS) emerged in China in 2002 and went on to infect more than 8,000 people worldwide and killed more than 800. Indonesia was not affected.