More than 150 indigenous people are occupying one of the construction sites for the huge Belo Monte hydro-electric dam across the Xingu River in the Brazilian Amazon, a local spokesman said Tuesday.

"They have for the past five days been occupying the Pimentel site where the river has already dried up and they are awaiting the arrival (Thursday) of the head of the Norte Energia consortium in charge of the works," said Cleanton Ribeiro, a spokesman for the indigenous missionary Council Cimi.

"They are demanding demarcation of their lands, the expulsion of invaders, an improved health system and running water," he added.

"They no longer believe in the promises made (by the consortium) and say they will leave only when concrete steps begin," Ribeiro noted.

The activists include members of the Xicrin, Juruna, Aarara, Aawete, Assurini and Parakanawa tribes.

Norte Energia says some 17 socio-economic and environmental projects worth $117 million have already been launched in the region, the business daily Valor reported Monday.

The third largest dam in the world, the 11,200-megawatt Belo Monte is one of several hydro projects meant to provide Brazil with clean energy for its fast-growing economy.

Work began a year ago, despite fierce opposition from local people and green activists.

Indigenous groups fear the dam will harm their way of life while environmentalists have warned of deforestation, greenhouse-gas emissions and irreparable damage to the ecosystem.

Belo Monte is expected to flood an area of 500 square kilometers (200 square miles) along the Xingu and displace 16,000 people, according to the government, although some NGOs put the number at 40,000 displaced.

The federal government plans to invest a total $1.2 billion to assist the displaced, by the time the dam is completed in 2019.