A major Rio hydroelectric power plant was switched off after water levels slipped below an operational minimum following severe drought, Brazil's national grid told AFP on Thursday.

Water levels are also dwindling at three other plants that serve a region home to 16 million people, but an ONS spokesman denied there was any immediate threat to energy supplies.

"The hydroelectric plant was switched off Wednesday as there was no longer enough water to keep its turbines turning," the spokesman said of the shutdown at the largest of the four sites.

Rio is laboring under weeks of scant rainfall as stifling temperatures approach 40 degrees Celsius (104 Fahrenheit).

The plants receive their water from the Paraiba do Sul river, which flows through the states of Sao Paulo and Minas Gerais, as well as Rio.

Last month, the states' governors agreed to begin work on improving the infrastructure of the sites as the drought shows little sign of abating.

Last year, Sao Paulo, Brazil's most populous state, suffered its worst drought in 80 years.