Ethnic minorities have been undercounted in Bangladesh's latest census, Indigenous activists said Thursday, with implications for some of the poorest people in the country.

Bangladesh's population is 99 percent Bengalis, with a scattering of ethnic and Indigenous groups -- most of them Buddhists and Christians -- in the Muslim-majority country's northern hill districts.

Census results in July said those minorities amounted to 1.65 million people, one percent of the national total of 165 million.

But Sanjeeb Drong, general secretary of the Bangladesh Indigenous Peoples' Forum, said that his organisation estimated there were "around three million" -- almost twice as many.

Many ethnic minorities in remote areas had gone uncounted by census enumerators, he said.

"If you don't count the indigenous people, it is easy to ignore their demand for self determination, land rights, development budget and also human rights," he said.

The census found that Indigenous people were now a minority in two of the three districts of the Chittagong Hill Tracts, where ethnic groups waged a decades-long insurgency for self-rule.

The main rebel group signed a peace treaty with Dhaka in 1997, but a low-intensity conflict persists in some areas.

The 1.65 million figure was up slightly from the previous total in 2011, but included an additional 23 ethnic groups.

According to the figures, the population of the 27 groups included in the last census had actually declined over the period, said Philip Gain, of the Society for Environment and Human Development.

"There is a clear carelessness, neglect and lack of skills on the part of the enumerators and perhaps an ill intention to undercount the ethnic communities," he said.

Dipankar Roy, a senior official of the state-run Bangladesh Bureau of Statistics, which conducted the census, defended the survey and ruled out any possibility of deliberate undercounting.

"There will be a post enumeration check by a third party. If there is any undercount and overcounting, it will be adjusted," he told AFP.

Rohingya refugees were not included in the survey.

Nearly one million Rohingya fled to Bangladesh after a military offensive in neighbouring Myanmar five years ago and now live in sprawling camps near the border.