Families in Kenya were reportedly fleeing their homes and taking what possessions they could on Tuesday after police gathered near the Embobut forest to evict thousands of indigenous Sengwer people and others said to be threatening urban water supplies.

The forest, in the Cherangani hills near the town of Eldoret in western Kenya, has been the ancestral home of marginalised hunter-gatherer Sengwer communities for centuries, but the area has been invaded in the past 20 years by many thousands of other people seeking land to grow food.

The authorities claim the result has been wholesale destruction of the forest and the drying up of rivers that provide drinking water and irrigation for many nearby towns and villages. Exact numbers of people living in the forest are uncertain, but estimates range from 7,000 to 15,000.

"Reports reaching us from Sengwer community members in Embobut today tell of a chaotic situation as people are threatened and are in fear of their safety," a spokesman for the UK-based Forest Peoples Programme said. "Some families are fleeing their homes in fear of forceful evictions; 150 police and forest guards, including also 30 riot police, are massing to carry out the evictions from the three locations of Tangul, Kipsitono and Maron near the forest. More troops may join."

At least two successive Kenyan governments have threatened the Sengwer communities and forest squatters with evictions. In the past two years, many houses have been burned down in attempts to force people to move. In November 2013, President Uhuru Kenyatta went to Embobut and promised people money to move, but the offer has been widely ignored. The government deadline for moving from the forest expired last week.

But more than 40 Kenyan and international human rights and environment groups this week questioned the legality of the imminent evictions of the Sengwer and the way the Kenyan authorities have tried to remove indigenous people from forested land in the name of conservation.

"[This eviction] would violate the human rights of the indigenous Sengwer/Cherangany peoples, and their right to the customary sustainable use and conservation of biodiversity, if they are forcibly evicted from their ancestral lands and deprived of their own indigenous means of subsistence integral to their forest life, identity, their characteristic sources of food, water, health and shelter and to their cultural survival as a people," the groups said in a joint statement.

The Forest Peoples Programme added: "For many years the government has been trying to move the indigenous inhabitants of Embobut off their land by burning their homes. They have done this in the name of a 'fortress conservation' approach, which seeks to remove local people from their lands. As all pre-eminent conservation organisations now acknowledge, such an approach only ever makes the environmental situation worse, and adds a human rights disaster to the environmental crisis."

According to the Nairobi-based UN Environment Programme, the rolling Cherangani hills are among the country's five most important water catchment areas. Conservation of the upland areas is considered to be vital to prevent water shortages in cities.