COLOMBO (AFP) - The entire nation of Sri Lanka was left without power on Monday (Aug 17) in an hours-long outage following an "unspecified failure" at a major power plant, the state-run electricity board said.

Minister of Power Dullas Alahapperuma said a "technical issue" at the Kerawalapitiya power station just outside the capital of Colombo was the cause of the blackout.

The outage hit the entire nation of 21 million people at around midday, and although power returned to some parts of the country after six hours, Colombo was in darkness as night fell.

It is the worst disruption since March 2016, when the entire nation was without electricity for more than eight hours following a massive system breakdown.

"Steps being taken to restore the supply," Mr Alahapperuma said in a statement, without saying when power would be restored.

The power cut caused chaos on already congested roads in Colombo, with traffic lights not operating and police struggling to man key intersections.

Hospitals and other critical infrastructure in the country have power generators. The main airport has remained mostly shut because of the coronavirus pandemic.

Sri Lanka generates just over half of its electricity through thermal power. The remainder comes from hydro and wind power.

Kerawalapitiya is an oil-fired thermal power station with a 300MW capacity, about 12 per cent of the country's electricity demand.

Localised power failures are not uncommon but nationwide outages are rare.