South Korean nuclear officials came under fire Wednesday for failing to report a brief electricity failure at an ageing atomic power plant until more than a month after the incident.

The Korea Hydro and Nuclear Power Co (KHNP), operator of the country's atomic plants, said it had now halted the Gori-1 reactor near the southern port of Busan for an inspection on orders from state nuclear safety authorities.

The order followed a KHNP report that the reactor briefly lost mains power on February 9 and that the emergency generator failed to kick in.

The power cut forced the reactor's cooling water to stop circulating. The electricity supply resumed 12 minutes later but KHNP did not report the incident to the safety authorities until over a month later.

The Korea JoongAng Daily said it was the first such power failure since the Gori-1 reactor, the country's first, began operating in 1978. It was unclear whether the brief outage caused the temperature of the fuel rods to rise.

Last year's meltdown at Japan's Fukushima plant occurred when a tsunami knocked out electrical pumps cooling reactors.

A KHNP spokesman acknowledged that his company failed to report the problem immediately in compliance with rules, but told AFP this was because "it was a minor accident and the problem was fixed fast".

Knowledge Economy Minister Hong Suk-Woo in a statement Wednesday offered a public apology for the power failure and promised a thorough investigation.

He blamed human error for the total power loss.

Park Hyun-Shik, an official from the Korea Institute of Nuclear Safety, said the incident could have become more serious had the power not restarted.

"What should be considered as an even bigger problem is that the operator tried to hide what had happened," Park told the Korea JoongAng Daily.

Professor Kim Yong-Soo of Seoul's Hanyang University in Seoul told AFP that system failures may increase because a growing number of South Korean reactors are close to the end of their life cycles.

South Korea relies on 20 nuclear reactors to meet about 35 percent of its electricity needs. The government has vowed to stick to the programme despite heightened concern following the nuclear disaster at Fukushima.

The top opposition leader vowed Monday to gradually reduce reliance on nuclear energy if her party wins next month's general election.

"I want to see the Fukushima crisis as the last shocking warning for mankind about problems with nuclear power plants," said Han Myeong-Sook, chairwoman of the Democratic United Party.