South Korea's nuclear watchdog said Wednesday it would carry out a rigorous inspection of all the country's 23 reactors after a scandal involving unvetted components fuelled public fears about nuclear safety.

On Monday the government said it was shutting down two reactors fitted with thousands of parts that had been provided with forged quality and safety warranties.

The two affected units at the Yeonggwang nuclear complex may remain offline until early January, causing severe power shortages during the harsh winter.

"All 23 nuclear reactors in operation and five other reactors under construction will be scrutinised," Shim Eun-Jung, spokeswoman for the Nuclear Safety and Security Commission, told AFP.

"This kind of an extensive probe is rare," she said, adding that the commission had set up an investigation team of nearly 60 private and state experts.

The watchdog will also look into possible collusion by officials in the state-run Korea Hydro and Nuclear Power (KHNP) with the suppliers who faked the quality certificates.

KHNP president Kim Kyun-Seop told a parliamentary hearing Wednesday that he would willingly resign once the situation had been resolved.

"I'm ready to step down any time... I will not cling to my post. I will take all the responsibility," Kim said.

Kim Joong-Kyum, the CEO of power utility Korea Electric Power Corp which owns KHNP -- had also offered to resign, the economy ministry said.

All parts supplied for use in South Korea's 23 reactors, which meet more than 35 percent of national electricity needs, require quality and safety warranties from one of 12 international organisations designated by Seoul.

Officials said eight suppliers faked 60 warranties covering nearly 7,700 items that had been provided at a cost of 820 million won ($750,000).

Of the total, more than 5,200 parts have been used in five reactors -- 99 percent of them in the two Yeonggwang units closed on Monday.

Knowledge Economy Minister Hong Suk-Woo said Monday the "non-core" components posed no safety threat.

But he warned of a "dramatic" drop in national power reserves if the two reactors are not brought back online as scheduled.

The government has vowed to stick to its nuclear power programme despite public concerns arising from last year's nuclear disaster in Japan. South Korea plans to build an additional 16 reactors by 2030.

Doubts over safety standards were fuelled in May when five senior engineers were charged with trying to cover up a potentially dangerous power failure at the country's oldest nuclear plant.

The Gori-1 reactor, built in 1978 near the southern city of Busan, briefly lost mains power on February 9 and the emergency generator failed to kick in. The power cut caused cooling water to stop circulating.

Last month, authorities temporarily shut down two 1,000-megawatt reactors at separate nuclear plants after system malfunctions. These were also blamed for another reactor at Yeonggwang being tripped into automatic shutdown in July.