SEOUL (AFP) - South Korean Prime Minister Chung Hong Won vowed on Friday to mete out severe penalties to anyone involved in a forged documentation scandal that has shut down a host of nuclear reactors.

"It is a grave crime that angers both heaven and human beings," Mr Chung told officials at a government policy coordination meeting.

South Korea shut down two reactors on Tuesday and delayed the scheduled start of operations at two more, prompting government warnings of "unprecedented" power shortages.

The move, part of a widening investigation into a scandal involving parts provided with fake safety certificates, means 10 of the South's 23 nuclear reactors are currently offline for various reasons.

"Those found to be involved in wrongdoing or corruption must be sternly punished by the law, regardless of their rank and status," Mr Chung said.

The Ministry of Trade, Industry and Energy is expected to announce a package of measures later on Friday aimed at reducing energy consumption in the summer.

The ministry has warned that blackout alerts, triggered automatically when power reserves dip below a certain level, were highly likely and power shortages could be "very serious" in August.

At proper capacity, South Korea's nuclear reactors supply more than 35 per cent of national electricity needs.

All parts supplied for use in the reactors require quality and safety warranties from one of 12 international organisations designated by Seoul.

Last year, officials said eight suppliers were found to have faked warranties covering thousands of items used in a number of reactors. Earlier this month, six nuclear engineers and suppliers were jailed for their part in the scandal.

Although the suspect parts were "non-core" components that presented no public safety risk, the authorities instigated an inspection of all reactors nationwide.

South Korea's nuclear sector has been dogged by a series of malfunctions, forced shutdowns and corruption scandals that have undermined public confidence already shaken by the 2011 Fukushima nuclear disaster in Japan.

In May last year, five senior officials of the state-run Korea Hydro & Nuclear Power Co. were charged with trying to cover up a potentially dangerous power failure at the country's oldest Gori-1 reactor.

Despite increasing public concern, the government has vowed to push ahead with its nuclear power programme, and plans to build an additional 16 reactors by 2030.