A six-hour power failure across Sarawak caused chaos and plunged more than two million people into darkness.

The electricity outage hit on Saturday evening, darkening cities and towns across the state and causing massive traffic jams as signals failed.

"The state government wants to find out the source of the massive blackout. We need to know exactly," Deputy Chief Minister George Chan told The Star newspaper, describing the blackout as unprecedented in the state.

A Sarawak Electricity Supply Corporation official told the daily that it believed the cause of the outage was a fault along the main power grid between the towns of Sibu and Bintulu.

Sarawakians have been warned that power supply is still not stable and further blackouts may occur.