The indigenous Ache people in Paraguay threatened Tuesday to open fire with bows and arrows on farmers who invade their land to plant marijuana or cut down trees.

"This is the last warning. They need to understand the gravity of the situation," Ache leader Martin Achipurangi told AFP by phone from the eastern-central department of Canindeyu.

The Ache live on about 6,400 hectares (16,000 acres) of land, some of the last of Paraguay's once sprawling virgin forests.

They survive mainly by farming, hunting and fishing, but have seen increasing incursions by marijuana growers and illegal loggers.

Achipurangi said he had sent an envoy to the capital, Asuncion, to address the issue with interior ministry officials.

"Here they don't pay any attention to us, neither prosecutors nor the police," he said, accusing "opportunists and fraudsters" of seeking to occupy the Ache's land.

Researchers from environmental group WWF last week found that the Ache reserve was dotted with cannabis farms, charred vegetation and logging trucks.

Achipurangi said he had ordered men in his community to begin making bows and arrows, including for children.

"We are giving the authorities three weeks to act. Otherwise, we're going to use our own means to defend our community," he said.

The Ache were the last indigenous people to emerge from the rainforest in Paraguay, in the 1960s and 1970s.

Since then, an estimated 4,000 of them have died of disease or been killed, reducing the population to an estimated 2,000 people.